Source: https://fullstackelectronics.com/experiments/09-the-voltage-divider

# Experiment 9: The Voltage Divider

Make both of the 555's references on the board from three equal resistors. Then lower the supply and measure how they follow it.

Experiments 7 and 8 tested each comparator against power supply output 2, set by hand to 3.33 V or 1.67 V. That was a temporary reference. Inside the 555, three equal resistors run in series from the supply pin to ground. The upper join between them sits at 2/3 of the supply and is the threshold comparator's reference. The lower join sits at 1/3 and is the trigger comparator's reference. A chain like this is called a [voltage divider](https://fullstackelectronics.com/reference/glossary.html.md#voltage-divider). Both references are fixed shares of the supply, so they follow it when it changes, as Figure 9-1 shows. That keeps the 555's timing the same at any supply. In the finished 555 the timing capacitor charges from 1/3 to 2/3 of the supply, and that climb takes the same time whatever the supply is. The theory box “Why the timing ignores the supply” explains this. It comes after the tap readings. This experiment builds the divider, measures it at 5.00 V and 4.50 V, and connects it to both comparators.

![Bar chart on a 0 to 5 volt scale with four pale bars, each running from 0 volts up to its supply. Two red marks across each bar show the two references, with their share of the supply beside them. Left pair, references from power supply output 2: with a 5.00 volt supply, 3.33 volts is 0.67 of the supply and 1.67 volts is 0.33; with a 4.50 volt supply, the references stay at 3.33 and 1.67 volts, which are 0.74 and 0.37 of the supply. Right pair, references from the divider: 3.33 and 1.67 volts at 5.00 volts, and 3.00 and 1.50 volts at 4.50 volts, 0.67 and 0.33 of the supply in both cases.](https://fullstackelectronics.com/experiments/img/09-fixed-vs-divider.svg)

**Figure 9-1:** The two references on a 5.00 V and a 4.50 V supply.

### You will need

- [ ] 3×10 kΩ resistor, 1 % metal film, for R1 to R3. A few spares let you choose three that match.
- [ ] The two 1 kΩ resistors from Experiment 8, which come off the threshold comparator's inputs and go back on as the ramp's resistors
- [ ] 1×Crocodile-clip lead
- [ ] The board from Experiment 8: the threshold comparator in columns 3 to 14 and the trigger comparator in columns 16 to 27
- [ ] Jumper wire
- [ ] Bench power supply: one output at 5.00 V with a 50 mA current limit to power the board (the 5 V supply in the steps), adjustable down to 4.50 V. Power supply output 2 is not needed after the first steps.
- [ ] Multimeter
- [ ] Oscilloscope with three channels and two probes. With two channels, look at one output at a time.
- [ ] Function generator
- [ ] BNC cable, BNC T-piece and a BNC lead ending in mini-hook clips

## Three equal resistors

Before the divider goes on the board, here is how three resistors make the two references. Figure 9-2 is the circuit. R1, R2 and R3 are 10 kΩ resistors in series from +5 V to ground. A tap is a join between two resistors where a voltage is taken off: the 2/3 tap between R1 and R2, and the 1/3 tap between R2 and R3.

![Schematic of the divider. A +5 volt symbol at the top feeds R1, then R2, then R3, each 10 kilohms, in a vertical chain down to a ground symbol. The join of R1 and R2 leaves to the right as the 2/3 tap, marked 3.33 volts. The join of R2 and R3 leaves to the right as the 1/3 tap, marked 1.67 volts. Four numbered markers match the steps below. Marker 1: a current arrow pointing down beside R1. Marker 2: beside R3. Marker 3: the 1/3 tap. Marker 4: the 2/3 tap.](https://fullstackelectronics.com/experiments/img/09-divider.svg)

**Figure 9-2:** The divider, with the four steps marked. The numbers match the list below.

1.  **One current flows down the chain.** Nothing else connects to it yet, so the same current passes through R1, R2 and R3. The three resistors add up to 30 kΩ, so the 5.00 V supply drives 5.00 V / 30 kΩ = 0.167 mA through them.
2.  **Each resistor drops the same voltage.** The voltage across a resistor is its current times its resistance: 0.167 mA × 10 kΩ = 1.67 V. The three resistors carry the same current and have the same resistance, so each one takes a third of the supply.
3.  **The 1/3 tap is one drop above ground:** 1.67 V, across R3 alone.
4.  **The 2/3 tap is two drops above ground:** 3.33 V, across R2 and R3.

At 4.50 V each resistor drops 1.50 V, and the taps move to 3.00 V and 1.50 V. The shares depend only on how the three resistances compare, so the three have to match. Their common value sets two smaller things: the current through the chain, and how far a tap's voltage moves when something draws current from it. Later in this experiment each comparator input draws a few microamps from its tap. With 10 kΩ resistors that moves a tap by a few tens of millivolts, and Figure 9-7 shows that shift.

#### Is the 555 named after these resistors?

The NE555's divider uses 5 kΩ for each of its three resistors, half the value on this board. Several books say the chip takes its name from them: 5, 5, 5. Its designer said otherwise. Hans Camenzind designed the 555 for Signetics, and in an oral-history interview for the Transistor Museum he said the number was chosen arbitrarily. Art Fury, Signetics' marketing manager, expected the circuit to sell well and picked "555" from the company's 500 series of part numbers. The three resistors do have the same value, and the match with the name is a coincidence.

## Build the divider

The divider replaces power supply output 2, so the bench connections from Experiment 8 come off first. That experiment left power supply output 2 and the generator on the threshold comparator, each lead clipped onto a 1 kΩ resistor.

Figure 9-2 draws the divider with +5 V at the top. The +5 V rail of this board is at the bottom, so on the board the divider stands upside down. It goes in column 15, between the two comparators. That column is free in both blocks, so R2 can cross the centre channel. The lower strip of column 15 (rows f to j) becomes the 2/3 tap, and the upper strip (rows a to e) the 1/3 tap. Figure 9-3 shows the divider and where the multimeter goes.

[![The breadboard drawn 40 columns wide, with both comparators faded and no input resistors. In colour, three 10 kilohm resistors in column 15. R1 stands from 15j over the bottom ground rail to the outer bottom rail, +5 volts. R2 crosses the centre channel from 15f to 15e. R3 stands from 15a over the top +5 volt rail to the outer top rail, ground. Red rings mark the multimeter's red probe at 15c, labelled 1/3 tap, and at 15h, labelled 2/3 tap. A blue ring marks its black probe on the bottom ground rail at column 26.](https://fullstackelectronics.com/experiments/img/09-breadboard-divider.svg)](https://fullstackelectronics.com/experiments/img/09-breadboard-divider.svg)

**Figure 9-3:** The divider in column 15, and the multimeter's probe points. The lower strip of column 15 is the 2/3 tap, and the upper strip is the 1/3 tap.

1.  **Switch everything off:** the 5 V supply, power supply output 2 and the generator.
2.  **Remove power supply output 2.** Unclip its positive lead from the 1 kΩ in 13j, and take its negative lead off the top ground rail at column 16. The divider replaces it.
3.  **Unclip the generator's red lead** from the 1 kΩ in 10j. Leave its black lead on the top ground rail at column 7.
4.  **Pull both 1 kΩ resistors out** of 10j and 13j. Keep them: they go back on the comparator inputs later in this experiment.
5.  **Match the three 10 kΩ resistors.** Set the multimeter to resistance and measure each resistor before it goes on the board. Choose three that are within 1 % of each other, which is 100 Ω.
6.  **Fit R1** from 15j to the bottom +5 V rail, the outer one. It reaches over the ground rail.
7.  **Fit R2** from 15f to 15e, across the centre channel.
8.  **Fit R3** from 15a to the top ground rail, the outer one. It reaches over the +5 V rail.

## Measure the taps

The divider is complete, and nothing takes current from its taps yet. The taps are shares of the voltage that reaches the board, so set the supply by the multimeter.

1.  **Switch on the 5 V supply** only. The generator stays off.
2.  **Measure the supply.** Put the black probe on the bottom ground rail at column 26 and the red probe on the bottom +5 V rail, the outer one. Adjust the supply until the multimeter reads 5.00 V.
3.  **Measure the 2/3 tap.** Move the red probe to 15h. Expect 3.33 V.
4.  **Measure the 1/3 tap.** Move the red probe to 15c. Expect 1.67 V.
5.  **Lower the supply to 4.50 V,** measured on the bottom +5 V rail as before. Measure both taps again. Expect 3.00 V and 1.50 V.

If both taps read close to 0 V, R1 is on the inner bottom rail, which is ground. If both read close to the supply, R3 is on the inner top rail, which is +5 V. Switch off and move that resistor's lead to the outer rail.

The table gives the expected values and the readings from my first build of this circuit. The number in brackets beside each reading is that reading divided by the supply.

| Supply | 2/3 tap, expected | 2/3 tap, my first build | 1/3 tap, expected | 1/3 tap, my first build |
|----|----|----|----|----|
| 5.00 V | 3.333 V | 3.354 V (0.671) | 1.667 V | 1.695 V (0.339) |
| 4.50 V | 3.000 V | 3.018 V (0.671) | 1.500 V | 1.525 V (0.339) |

Both taps fell with the supply and kept their share: 0.671 and 0.339 at both supplies. The small offset from 0.667 and 0.333 is the three resistors differing by a fraction of a percent. Set the supply back to 5.00 V, measured on the bottom +5 V rail.

#### Why the timing ignores the supply

The table showed both taps keeping their share of the supply. Here is why that keeps the 555's timing the same. In the finished 555, a resistor charges the timing capacitor from the supply, and the capacitor's voltage climbs from the 1/3 tap to the 2/3 tap. The current through the resistor depends on the voltage across it: the supply minus the capacitor's voltage. Call that the gap. On a 5.00 V supply the gap starts at 3.33 V and ends at 1.67 V. On 4.50 V it starts at 3.00 V and ends at 1.50 V. On both supplies the climb ends when the gap has halved.

As the gap shrinks, the current shrinks with it, so the gap falls by the same fraction in each equal stretch of time, like a half-life. Halving takes the same time whatever size the gap starts at: 0.69 RC on both supplies, as Figure 9-4 shows. With references fixed at 3.33 V and 1.67 V on a 4.50 V supply, the gap would fall from 2.83 V to 1.17 V, which is less than half, and the climb would take 28 % longer.

![Graph of capacitor voltage against time, from 0 to 1.6 RC. A dark curve starts at 1.67 volts and rises towards a dashed line at 5.00 volts. A green curve starts at 1.50 volts and rises towards a dashed line at 4.50 volts. Dotted lines mark 3.33 volts and 3.00 volts. A vertical dotted line at 0.69 RC passes through the point where the dark curve reaches 3.33 volts and the point where the green curve reaches 3.00 volts.](https://fullstackelectronics.com/experiments/img/09-charging.svg)

**Figure 9-4:** The capacitor's climb from 1/3 to 2/3 of the supply, on 5.00 V and on 4.50 V. Both curves reach 2/3 of their supply at 0.69 RC. These curves are calculated.

Where the 0.69 RC and the 28 % come from optional

Call the supply $V$, the capacitor's voltage $v$, and the gap between them $u$:

$$u = V - v$$

The current through the resistor is the gap divided by the resistance. All of it flows into the capacitor, and a current into a capacitor raises its voltage at a rate of current divided by capacitance:

$$\frac{dv}{dt} = \frac{i}{C} = \frac{u}{RC}$$

The supply does not change, so the gap shrinks exactly as fast as the capacitor's voltage rises:

$$\frac{du}{dt} = -\frac{dv}{dt} = -\frac{u}{RC}$$

A quantity that shrinks at a rate proportional to its own size decays exponentially. If the gap is $u_0$ at the start, then after a time $t$ it is

$$u = u_0\, e^{-t/RC}$$

The climb ends when the gap has fallen to $u_1$. Divide both sides by $u_1\,e^{-t/RC}$ to get

$$e^{t/RC} = \frac{u_0}{u_1}$$

and take the natural logarithm of both sides:

$$t = RC \ln \frac{u_0}{u_1}$$

The time depends only on the ratio of the two gaps. With the divider, the climb starts at $V/3$ and ends at $2V/3$:

$$u_0 = V - \tfrac{1}{3}V = \tfrac{2}{3}V \qquad u_1 = V - \tfrac{2}{3}V = \tfrac{1}{3}V$$

The supply cancels in the ratio:

$$t = RC \ln \frac{\tfrac{2}{3}V}{\tfrac{1}{3}V} = RC \ln 2 \approx 0.693\,RC$$

With references fixed at 1.67 V and 3.33 V on a 4.50 V supply, the gaps are

$$u_0 = 4.50\ \mathrm{V} - 1.67\ \mathrm{V} = 2.83\ \mathrm{V} \qquad u_1 = 4.50\ \mathrm{V} - 3.33\ \mathrm{V} = 1.17\ \mathrm{V}$$

so the climb takes

$$t = RC \ln \frac{2.83}{1.17} = RC \ln 2.43 \approx 0.887\,RC$$

That is 0.887 / 0.693 = 1.28 times the time with the divider: 28 % longer. The 0.693 is the number in the NE555's timing formula in Experiment 1.

## Connect the references to the comparators

The divider makes both references, and they follow the supply. It can now replace power supply output 2 at both comparators. In the finished 555, each comparator compares its reference with the timing capacitor's voltage. [Experiment 12](https://fullstackelectronics.com/experiments/12-the-swap.html.md) adds the capacitor. Until then a triangle from the generator stands in for the capacitor's voltage, and this page calls it the ramp.

Which input takes the ramp depends on the latch. The latch of [Experiment 11](https://fullstackelectronics.com/experiments/11-the-sr-latch.html.md) is switched by high levels, so each comparator's OUT has to go high when the ramp leaves the band between the taps: the threshold comparator's above 2/3, the trigger comparator's below 1/3. Experiments 7 and 8 measured both comparators with OUT high while V+ is below V−. The trigger comparator keeps the ramp on V+ and takes the 1/3 tap on V−, so its OUT is high while the ramp is below 1/3. The threshold comparator needs its inputs swapped from Experiment 7: the 2/3 tap goes on V+ and the ramp on V−, so its OUT is high while the ramp is above 2/3. Figure 9-5 draws both.

The ramp reaches each comparator through its own 1 kΩ, as in Experiments 7 and 8, because a base at the end of an instrument's lead can oscillate. The taps reach their bases on short jumpers and need no resistor.

![Schematic of both comparators on the divider. The divider stands at the left: +5 volts, then R1, R2 and R3, 10 kilohms each, down to ground at the bottom. A ramp terminal comes in from the left just above ground and hops the divider's ground wire. Two blocks stand to the right: the threshold comparator, Q1 to Q7, above, and the trigger comparator, Q8 to Q14, below. The 2/3 tap runs to the threshold comparator's V+. The ramp rises on a vertical wire and reaches the threshold comparator's V− through a 1 kilohm resistor. The 1/3 tap runs across the ramp's wire, hopping it, to the trigger comparator's V−. The ramp reaches the trigger comparator's V+ through a second 1 kilohm resistor. Threshold OUT and trigger OUT leave on the right.](https://fullstackelectronics.com/experiments/img/09-front-end.svg)

**Figure 9-5:** Both comparators on the divider. The threshold comparator takes the 2/3 tap on V+ and the ramp on V−. The trigger comparator takes the ramp on V+ and the 1/3 tap on V−.

Figure 9-6 shows those connections on the board. The 2/3 tap reaches Q1's base at 10j on a diagonal jumper from 15i, and the 1/3 tap reaches Q9's base on a short jumper from 15b to 17b. The trigger's 1 kΩ stands in 20a, Q8's base column, as in Experiment 8. The threshold's 1 kΩ stands in 13j, Q2's base column, as in Experiment 7. The ramp goes to the free leg of each, and Figure 9-6 gives both free legs the same label.

[![The breadboard with both comparators and the divider faded. In colour: a yellow jumper on a shallow diagonal from 15i to 10j, labelled 2/3 tap to threshold V+, and a short yellow jumper from 15b to 17b, labelled 1/3 tap to trigger V−. A 1 kilohm resistor stands in 13j and leans down over the bottom rails. A second 1 kilohm resistor stands in 20a and rises over the top rails. A red ring marks the free leg of each resistor, and both rings have the same label: ramp, triangle 1 to 4 volts, 100 hertz. Rings mark the generator's black lead on the top ground rail at column 7, the oscilloscope channel 2 tip on the threshold OUT at 11e, the channel 3 tip on the trigger OUT at 16f, and their grounds on the bottom ground rail at columns 20 and 28.](https://fullstackelectronics.com/experiments/img/09-breadboard-ramp.svg)](https://fullstackelectronics.com/experiments/img/09-breadboard-ramp.svg)

**Figure 9-6:** Both comparators on the divider and on one ramp. Each ramp input has its own 1 kΩ.

1.  **Switch off** the 5 V supply.
2.  **Fit the threshold's 1 kΩ.** Push one leg into 13j, in Q2's base column, and let it lean down over the bottom rails.
3.  **Fit the trigger's 1 kΩ.** Push one leg of the other 1 kΩ into 20a, in Q8's base column, and let it stand up over the top rails.
4.  **Connect the 2/3 tap to the threshold comparator's V+.** Fit a jumper from 15i to 10j. It passes just above 13j. Column 10 of the lower block is Q1's base.
5.  **Connect the 1/3 tap to the trigger comparator's V−.** Fit a jumper from 15b to 17b. Column 17 of the upper block is Q9's base.
6.  **Connect the ramp.** Clip the generator's red lead onto the free leg of the 1 kΩ in 20a. Connect the two free legs with the crocodile-clip lead.
7.  **Connect the probes.** Put the oscilloscope channel 2 tip on 11e, the threshold comparator's OUT, with its ground on the bottom ground rail at column 20. Put the channel 3 tip on 16f, the trigger comparator's OUT, with its ground on the bottom ground rail at column 28.

If a comparator switches near the other comparator's reference, its tap jumper is in the wrong strip of column 15. The 2/3 tap is the lower strip, rows f to j. The 1/3 tap is the upper strip, rows a to e. If the threshold comparator's OUT is high while the ramp is low, its inputs are the Experiment 7 way round: check that the 2/3 jumper ends in 10j and the 1 kΩ stands in 13j.

## Both comparators, one ramp

Both comparators now take their references from the divider and share one ramp. Together they are the 555's analogue front end, all made from the 5 V supply. The threshold comparator's OUT should be high while the ramp is above 3.33 V, and the trigger comparator's while it is below 1.67 V.

1.  **Set the generator** to a triangle from 1 V to 4 V at 100 Hz: a high level of 4 V and a low level of 1 V. Keep its load setting at HiZ. This range crosses both taps and stays inside the supply, at 5.00 V and at 4.50 V.
2.  **Set the oscilloscope:** channels 1, 2 and 3 at 1 V/div, with their ground markers at the same height near the bottom of the screen, and the timebase at 2 ms/div. Trigger on channel 1, rising, at 2.5 V.
3.  **Switch on** the 5 V supply and the generator.
4.  **Put the Y cursors on channel 1** at the two points of the ramp where the outputs switch.

![Oscilloscope with channels 1, 2 and 3 at 1 volt per division and the same ground marker, 2 milliseconds per division, triggered on channel 1 rising at 2.5 volts, supply 5.00 volts. The yellow ramp, a triangle from 1 to 4 volts at 100 hertz, rises and falls across the screen. The magenta trace, the threshold comparator's OUT at 11e, sits near 5 volts while the ramp is above about 3.36 volts and near 0 volts while it is below. The cyan trace, the trigger comparator's OUT at 16f, sits near 5 volts while the ramp is below about 1.66 volts and near 0 volts while it is above. Between the two levels both outputs are low. All traces are thin. Y cursors on channel 1 read Y1 1.66 volts and Y2 3.36 volts, a difference of 1.70 volts.](https://fullstackelectronics.com/experiments/img/09-front-end-5v.png)

**Figure 9-7:** The ramp in yellow, the threshold comparator's OUT in magenta and the trigger comparator's OUT in cyan, on a 5.00 V supply. The cursors read 1.66 V and 3.36 V.

The two outputs are high at opposite ends of the ramp, and both are low between the taps. In the finished timer, the capacitor moves only in that band, and the two high levels at its edges are what Experiment 11's latch needs. The switching points can sit a few tens of millivolts from the tap readings in the table. Each comparator input now draws a few microamps from its tap, and with 10 kΩ resistors that moves the tap by a few tens of millivolts.

The tap measurements showed the divider following the supply on its own. The last measurement checks the same with the comparators connected: at 4.50 V the switching points should move with the taps, to 3.00 V and 1.50 V.

1.  **Lower the 5 V supply to 4.50 V,** measured with the multimeter on the bottom +5 V rail. Keep the generator and the oscilloscope as they are.
2.  **Move the Y cursors** to the new switching points.

![The view of Figure 9-7 with the supply lowered to 4.50 volts: channels 1, 2 and 3 at 1 volt per division with the same ground marker, 2 milliseconds per division, triggered on channel 1 rising at 2.5 volts. The yellow ramp, a triangle from 1 to 4 volts at 100 hertz, rises and falls across the screen. The magenta trace, the threshold comparator's OUT at 11e, sits near 4.5 volts while the ramp is above about 3.00 volts and near 0 volts while it is below. The cyan trace, the trigger comparator's OUT at 16f, sits near 4.5 volts while the ramp is below about 1.50 volts and near 0 volts while it is above. Each output is high for longer than in Figure 9-7, and the band where both are low is narrower. Y cursors on channel 1 read Y1 1.50 volts and Y2 3.00 volts, a difference of 1.50 volts.](https://fullstackelectronics.com/experiments/img/09-front-end-4v5.png)

**Figure 9-8:** The measurement of Figure 9-7 on a 4.50 V supply. The cursors read 1.50 V and 3.00 V.

Figure 9-8 shows both switching points moving down with the taps. Set the supply back to 5.00 V when you have finished.

## The flaw list

No flaw joins the list. Every flaw on it is fixed or reduced.

| \# | Flaw | What it means | Found | Status |
|----|----|----|----|----|
| 1 | No reference input | The threshold is the transistor's own 0.6 V. Nothing outside the circuit sets it. | [Experiment 3](https://fullstackelectronics.com/experiments/03-the-almost-comparator.html.md) | Fixed in [Experiment 4](https://fullstackelectronics.com/experiments/04-the-differential-pair.html.md): the differential pair |
| 2 | Wide transition | The output needs a band of input to travel from high to low: about half a volt with one transistor, about a quarter of a volt with the pair. | [Experiment 3](https://fullstackelectronics.com/experiments/03-the-almost-comparator.html.md) | Fixed in [Experiment 6](https://fullstackelectronics.com/experiments/06-the-active-load.html.md): the active load |
| 3 | Thermal drift | The base-emitter voltage falls about 2 mV per degree, and the threshold falls with it. | [Experiment 3](https://fullstackelectronics.com/experiments/03-the-almost-comparator.html.md) | Reduced in [Experiment 4](https://fullstackelectronics.com/experiments/04-the-differential-pair.html.md): two matched transistors drifting together |
| 4 | Tail current follows the common-mode level | A resistor tail passes more current as both inputs rise, and the output moves with it. | [Experiment 4](https://fullstackelectronics.com/experiments/04-the-differential-pair.html.md) | Fixed in [Experiment 5](https://fullstackelectronics.com/experiments/05-the-current-mirror.html.md): the current mirror tail |
| 5 | Output low trapped a diode drop below the reference | The output's low level sits a diode drop below the reference instead of at ground. | [Experiment 6](https://fullstackelectronics.com/experiments/06-the-active-load.html.md) | Fixed in [Experiment 7](https://fullstackelectronics.com/experiments/07-the-output-stage.html.md): the PNP output stage |

## Quick check

Three questions on this experiment: what the divider's resistors must have in common, what the taps do when the supply falls, and why the threshold comparator's inputs were swapped.

1\. The divider's three resistors have to match because…

matched resistors take the least current from the supply

each resistor then takes the same share of the supply, a third

matched resistors warm up evenly and do not drift

The same current flows through all three. Equal resistors drop equal voltages, so the taps sit at exactly 1/3 and 2/3 of the supply. The common value sets the current and how far a small load moves a tap, but it does not change the shares.

2\. The supply falls from 5.00 V to 4.50 V. The taps…

stay at 3.33 V and 1.67 V

fall by 0.50 V each, to 2.83 V and 1.17 V

fall to 3.00 V and 1.50 V, still 2/3 and 1/3 of the supply

Each tap is a fixed share of the supply. At 4.50 V, 2/3 is 3.00 V and 1/3 is 1.50 V. A reference set on power supply output 2 would have stayed at 3.33 V and 1.67 V.

3\. The threshold comparator takes the 2/3 tap on V+ and the ramp on V− because…

its OUT is high while V+ is below V−, so it is then high while the ramp is above 2/3

an NPN pair cannot work with its reference on V−

the 2/3 tap is too weak to drive the V− input

Both comparators have OUT high while V+ is below V−. With the tap on V+, the threshold comparator's OUT goes high when the ramp rises above 2/3, which is the signal the latch needs. The trigger comparator keeps the ramp on V+, so its OUT is high below 1/3.

## Before moving on

Three matched resistors replaced power supply output 2. Their taps keep their share of the supply, so the 555's timing stays the same at any supply. The board now carries the 555's analogue half: the divider and both comparators.

Neither output remembers anything. When the ramp leaves the bottom of the band, the trigger comparator's OUT falls again, but the capacitor has to keep charging until it reaches 2/3. So the 555 needs one bit of memory that records which comparator was high last. That memory is a latch made of two logic gates. [Experiment 10: The NOR Gate](https://fullstackelectronics.com/experiments/10-the-nor-gate.html.md) builds the gate from transistors, and Experiment 11 joins two of them into the latch.