Circuits

The analog switch

An analog switch joins two points or keeps them apart, and a voltage decides which. With a capacitor after it, it grabs a voltage and keeps it: this page builds a sample and hold from one switch: the stepped random voltage that makes a filter jump.

New words

An analog switch is a resistor with two values (S1 a b c+ c-): small while its control voltage is high, huge while it is low.

A track and hold is a switch and a capacitor: while the switch is closed the capacitor follows the input, and when it opens it keeps the last voltage.

Switch Sample and Hold's Patch: an Oscillator set to Noise into SwitchHold, a copper circuit box with its schematic in small, an LFO wired into its clk; its out wired into a Filter's env; a keyboard below.

1Writing one

Write a switch as S, the two points it joins, then the two points its control voltage is measured between:

S1 a b c+ c- [on 100] [off 100M] [at 2.5V]

While c+ is more than at volts above c−, the switch is its on resistance; otherwise it is its off resistance. If you leave them out, they are 100 Ω, 100 MΩ and 2.5 V.
The control takes no current, so it can watch any point without changing it.
A circuit with a switch runs at 2× unless it says otherwise.

2Track, then hold

A switch and a capacitor make a track and hold: the capacitor follows the input while the switch is closed, and keeps its voltage when it opens. Close it with a square clock, and it follows for half of each cycle and holds for the other half.

For a sample and hold, close it for just a moment. In Switch Sample and Hold the clock comes through Cp, so only the moment the clock goes high lifts g, and Rp lets it fall back: the switch stays closed for only about 0.14 ms. On each clock it closes for that moment, C1 takes on the noise, and holds it until the next: a new random voltage on every eighth note, wired into a filter. U1 lets the filter read C1 without draining it.

SwitchHold · the parts list

# A sample and hold from one switch. S1 joins the noise at in to C1 while g is over 2.5 V. The clock's square comes# through Cp, so only the clock going high lifts g, and Rp lets it fall back: the switch closes for about 0.14 ms on each# clock, C1 takes on the noise, and holds it until the next. U1 lets the filter read C1 without draining it.circuit SwitchHold  setting input: 1 V  in clk: from 0 V to 5 V  Cp clk g 1n  Rp g gnd 100k  S1 in h g gnd  C1 h gnd 10n  U1 h out out opampend

SwitchHold's schematic: the switch S1 from in to h, C1 from h to ground, the op-amp U1 as a buffer to out; the clk input through Cp to g, with Rp to ground. — The same parts list, drawn: the switch is labelled with what closes it.

3Its limits

A switch acts where its control crosses at, even between two samples: SynthMaker takes that step again in pieces, and the switch moves at the crossing.
A circuit holds up to 4 switches, and a comparator written open counts as one.
It is ideal: two resistances and nothing else. None of a real switch chip's quirks: no click as it switches, no on resistance that moves with the signal, no gap as it changes over.

Try it

Open Switch Sample and Hold and hold a low key.
In the parts list, delete Cp and Rp and change S1 in h g gnd to S1 in h clk gnd: now it tracks the noise for half of every clock, and holds for the other half.
Change C1 h gnd 10n to 10u: the moment is too short to fill it, so the steps get small.
Set the clock's Sync from 1/8 to 1/4: a new step on every beat.

Open the example sound: Switch Sample and Hold ↗ (opens in a new tab)

Remember

A switch is S1 a b c+ c-: closed while its control voltage is over at, open otherwise.
A switch and a capacitor make a track and hold; close it for a moment and it is a sample and hold.
A switch acts exactly where its control crosses, so an oscillator reset through one keeps its pitch.