Four Modes, One Chip

The ISD1820 breakout nobody cares about · By Onion Madder · Mess O' Pedals

The ISD1820 is the ubiquitous blue voice-recorder module out of every talking greeting card, and near enough every tutorial wires it for three things: record, play once, play looping. But the chip has a fourth mode sitting right there on a pin nobody connects - feed-through, which sends the mic straight to the speaker in real time. Wire that one extra pin and your sampler is also a megaphone.

This is the breakout I built for my talk-box, and the feed-through is what takes this project from "noise box for one" to "annoy the neighbors for fun and profit".

⚠ Before you start tap to expand / collapse

Hot iron, obviously. Ventilate, don't breathe the flux smoke, park it in its stand every time.

Feed-through is live audio into a speaker. Mic and speaker in the same box is a feedback loop waiting to happen - point the speaker away from the mic, and start with the volume down before you go hunting for the squeal.

Mind your supply. The ISD1820 module is a 3–5V part. This is the one build in my series that does not want a big series pack - see Step 8.

What feed-through actually is

Straight from the datasheet: pull the FT pin high and the signal between MIC and MIC_REF runs through the chip's AGC, filter and speaker drivers and out of SP+ / SP−. It is not a recording, it is not a playback - it is the chip acting as a live amplifier, using the exact same audio chain playback uses. No extra amp, no extra parts. One wire.

Two things worth knowing about that pin before you wire it:

FT has a weak pull-down to Vss, so it idles LOW on its own. You don't need an external pull-down resistor, and leaving it unconnected is perfectly safe - that's just “feed-through off.”
Feed-through only engages if REC, PLAYE and PLAYL are all LOW at the same time. Pull FT high while any one of those is active and nothing comes out. No damage, no error, no clue - it just silently doesn't work. This is the single most important sentence on this page.

Quick reference - the four modes

Every mode is just a combination of four pins. H = held high, L = low, ↑ = rising edge.

Close-up of the ISD1820 module front. The header along the left edge is silkscreened top to bottom VCC, GND, FT, P-L, P-E, REC. The ISD chip sits in the centre marked ISD 1820PY, with the round electret microphone above it, a white two-pin connector marked SP1 on the right for the speaker, and three tactile buttons across the bottom labelled REC, PLAYE and PLAYL. — The pins you need are all on that one header. Note the silkscreen shortens them to **P-L** and **P-E**.

Mode state table
Mode	REC	PLAYE	PLAYL	FT	Control
Idle	L	L	L	L	-
Record	H	L	L	L	momentary (hold)
Sample (one shot)	L	↑	L	L	momentary (tap)
Loop	P-E pin 2 bridged to REC pin 1 — see Step 6				its own toggle
Megaphone	L	L	L	H	its own toggle

Module pins

VCC → 3–5V
GND → ground
REC → record while high
P-E (PLAYE) → play once on rising edge
P-L (PLAYL) → play while high
FT → feed-through while high
SP1 connector → speaker

Controls

Record → momentary to VCC
Sample → momentary to VCC
Loop → its own toggle
Megaphone → its own toggle
Loop = P-E pin 2 ↔ REC pin 1 (Step 6)

Loop and Megaphone — two switches, not one

Each mode gets its own switch. Megaphone is the straightforward one — a toggle between FT and VCC. Loop is the one that catches people out: it is a jumper between the second pin of P-E and the first pin of REC, which is not on any schematic for this board.

Wiring the two to opposite throws of one centre-off toggle sounds tidy, and it does not work — a shared switch can pull a pin up to VCC, but it cannot bridge two pins to each other, which is what the loop actually needs.

J1 is a 2×6 header. The rows are silkscreened down the left edge — VCC, GND, FT, P-L, P-E, REC — with a second column of pins beside them, plus separate FT and P-E pads to the right of the block. That two-column layout is why "jumper two pins" is the operative instruction here rather than "run a wire to VCC."

The underside of the ISD1820 module wired for the working build, held in helping hands, with hand-drawn annotations added. A green arrow labelled P-E points to a pad on the right-hand pad row, and the teal wire leaving it is traced in green down to a red toggle switch at the bottom. A yellow arrow labelled REC points to an adjacent pad, and its yellow wire is traced down to the same switch area. Orange and black power leads run off to a cylindrical lithium cell on the right. — The wiring that actually works, with the two pads called out: **P-E** (teal) and **REC** (yellow).

What you'll need

New to the parts? Each line notes how to spot it.

1× ISD1820 module - the little blue board with an electret mic and three tactile buttons. Spot it: silkscreen reads ISD1820, and the header pins are labelled VCC / GND / FT / P-L / P-E / REC down one edge - the header shortens the play pins to P-L and P-E even though the buttons say PLAYL and PLAYE
2× toggle switches - one for Loop, one for Megaphone. They each get their own switch; a single shared switch does not work
2× momentary push buttons - record and sample. Normally-open, so they only connect while pressed
1× speaker, 8 Ω - soldered to the pads the SP1 connector came off
A 3–5V supply - see the note in Step 8. Not a big series pack
Optional - 1× 500K potentiometer for the pitch mod (see Pitch control below). Spot it: marked B500K for a linear taper. Most guides say 100K; 500K sweeps a lot further
Hookup wire, enclosure, soldering iron, cutters, multimeter

The parts for the build laid out on a workbench: red-handled flush cutters on the left, a small cylindrical lithium cell marked 3.7V, the blue ISD1820 module with its J1 header silkscreened VCC GND FT P-L P-E REC, a potentiometer, two red mini toggle switches, two green panel-mount momentary push buttons, a large gold-framed round speaker, and orange-handled needle-nose pliers. A soldering station and spool of solder sit at the top left. — The parts list in one shot — **two** toggles (loop and megaphone get one each) and **two** momentary buttons (record and sample).

Step 1 Strip the board - the solder-to-desolder trick

The module ships with a tiny onboard speaker connector and an electret mic soldered in place. Both have to come off so you can run your own speaker and put the mic where you actually want it. This is the first thing you do, and it is where most people fight the board.

Do not try to suck the joint dry. The trick is the opposite of what it sounds like: add more solder. Fresh solder carries fresh flux, and that flux is what makes the old joint flow again. Pile a good blob across the pads, keep the iron moving between them so the whole thing stays molten, and the component essentially falls out. No braid, no pump, no lifted pads, no heat-soaking a part until it dies.

The other half of the trick is helping hands. Clip the board so both of your hands are free - one for the iron, one for the solder - because this only works while the joint is still liquid and you have about a second to lift the part clear.

The ISD1820 module flipped to its back, held in a helping-hands clip. The bare blue underside shows rows of tinned through-hole pads and the silkscreen letters MH. — Flip it over and clip it in. Everything happens from the back.

Desoldering the speaker connector: the board is clamped in a helping-hands clip while a soldering iron tip presses against a pad on the underside and a length of solder wire is fed straight onto the same joint, adding fresh solder rather than removing it. A soldering station with a red temperature display sits in the background. — Feeding fresh solder *onto* the joint. The flux in that new solder is the whole point.

The white two-pin SP1 speaker connector now removed and held between finger and thumb beside the board. The pads it came off are clear, wet-looking with pale flux residue, and undamaged. — SP1 lifted straight off. Pads intact, no braid needed.

The module repositioned in the helping-hands clip so the microphone pads face the iron, ready for the second removal. — Re-clip so the mic pads are the ones facing you.

Desoldering the electret microphone with the same method: iron tip on the pad from the back of the board while solder wire is fed onto the joint, with the board held steady in the helping-hands clip. — Same move for the mic - blob it up, keep it molten, lift.

The freed electret microphone capsule held between finger and thumb, its green backing board and solder terminals visible, next to the stripped module with both the mic and speaker connector now removed. — Both parts off and reusable - the mic goes back on flying leads wherever you want it.

Save both parts. That electret is the mic your megaphone mode listens through, so it is going straight back on a pair of wires - just somewhere you choose, rather than flat against the board. When you re-attach it, make sure its ground leg is properly grounded. An ungrounded mic is the difference between a megaphone and a howling mess - see Step 5.

Step 2 Power it up bare first

Before you wire a single control, give the module VCC and GND and a speaker, and use its onboard buttons to record and play something. The board ships with tactile buttons for REC / PLAYE / PLAYL precisely so you can prove it works before you commit to a panel. If it won't record now, no amount of switch wiring will fix it.

The stripped module held back-side-out in a helping-hands clip, its through-hole pads freshly tinned with small bright beads of solder ready to take wires. — Tin the pads first. A pre-tinned pad takes a wire in one touch instead of three.

Fingers holding an orange and a black hookup wire side by side, both stripped ends tinned silver so the strands are fused into a single solid tip. — Tin the wire ends too — fused strands go into a pad cleanly and don't splay.

The back of the module with the orange and black power leads now soldered onto the pads at the left-hand edge of the header. — Power in: orange to VCC, black to GND.

The module suspended in the helping-hands clip with its orange and black leads running out across the bench to a small cylindrical lithium cell. — Cell on the end of those leads — a single 3.7V lithium is the right supply for this board.

The front of the powered module held between finger and thumb. The small red D1 indicator LED on the top edge is lit, showing the board has power. — D1 lights: the board is alive before a single control is wired.

Step 3 Record - momentary to VCC

REC records for as long as it is held high. That is a hold-to-record button, not a tap-to-start one, so a normally-open momentary from REC to VCC is all it takes. Let go and the recording ends there.

Record button
Connect	To	Part
REC	VCC	momentary push button

Step 4 Sample - momentary to VCC

PLAYE is edge triggered: it fires on the rising edge and plays the whole message through, whether or not you keep holding it. That is your one-shot sample button - tap and it goes.

Sample button
Connect	To	Part
PLAYE	VCC	momentary push button

Step 5 Megaphone — its own toggle

The megaphone side is the simple one. A toggle between FT and VCC, and that is the whole connection. Flip it and the mic feeds the speaker live.

Megaphone switch
Connect	To	Part
One lug	FT	its own toggle
Other lug	VCC	its own toggle

FT idles low on its own, so the switch only ever has to make the connection — there is nothing to pull it back down when you switch off.

⚠ Ground the mic tap to expand / collapse

If the mic isn't grounded you will get mad feedback. Not the ordinary kind you fix by moving the speaker — a genuine howl the moment feed-through goes live.

It bites here specifically because Step 1 takes the electret off the board and puts it on flying leads. On the board its ground was handled for you. On wires, it is yours to get right. Check that ground leg before you go hunting for anything else.

Step 6 Loop — the jumper that isn't in any schematic

This is the one nobody documents. Loop is not P-L pulled up to VCC, and it is not anything you will find on a schematic for this board. It is a bridge between two specific pins on J1:

Loop switch
Connect	To	Part
One lug	P-E, second pin	its own toggle
Other lug	REC, first pin	its own toggle

Because J1 is a two-column header, every label on that edge covers two pins — which is the whole reason this is easy to miss. You are not connecting the P-E row to the REC row, you are bridging one specific pin from each, and they sit next to each other on the board. Get the wrong pin of either pair and nothing happens.

I found this by poking at the board and cross-reading other tutorials until something worked — it is not in the datasheet and it is not on any schematic I have seen. If you have only ever seen the three-mode version of this module, this jumper is the difference.

Loop and megaphone still can't both be on at once — the chip needs REC and the play pins low for feed-through, and this jumper drives them. Two separate switches simply means you decide which mode is live, rather than the hardware deciding for you.

Step 7 Speaker

Your speaker goes where the SP1 connector used to be - solder it straight to those two pads. The onboard amp drives it for playback and for feed-through - same chain, so megaphone mode comes out at the same level as your samples with no extra amplifier.

The module on the bench with green and yellow wire pairs running out to two components: the salvaged electret microphone capsule on the left, now on flying leads, and the large gold-framed speaker on the right. Orange and black leads run to the lithium cell behind. — Mic and speaker both out on flying leads — the mic can now live wherever you want it.

The finished wiring held up in one hand: the module with its D1 LED lit, the relocated electret microphone and the large speaker both connected on green and yellow leads, and the lithium cell hanging below. — Powered, mic relocated, speaker connected — this is the point it becomes a working box.

Step 8 Power

The ISD1820 module runs on 3–5V. Do not hand this one a big series pack the way you would a 555 or CMOS build - it is the exception in the series.

A single salvaged lithium cell at ~3.7V lands right in range, which makes the parallel vape pack the correct pack for this build: same 3.7V, double the runtime. That is exactly the low-power case that guide is built for. If you want more hours, add cells in parallel - not in series.

Step 9 If it misbehaves

Megaphone does nothing? Check the loop side isn't latched on. FT is ignored unless REC, PLAYE and PLAYL are all low. If you wired two separate toggles, this is why - go back to Step 6.
Megaphone howling? Check the mic ground first. An ungrounded electret feeds back hard the instant FT goes high, and no amount of moving the speaker will fix it. This is the most likely culprit if the noise started after you relocated the mic.
Still squealing once it's grounded? Ordinary feedback - mic and speaker are hearing each other. Turn it down, move the speaker, or point it away from the mic.
Records but plays back nothing? Confirm the speaker is on the old SP1 pads, and that REC actually went low again (a stuck button holds it in record).
Recording cuts off short? That's the chip's fixed message length, set by the onboard sampling resistor - which is also the thing you swap for a pot to get pitch control (see below).
Nothing at all? Back to Step 2 with the onboard buttons. Prove the board before you blame the panel.

Going further

Pitch control - and which pot to actually use

Swap the onboard sampling-frequency resistor R4 for a potentiometer and the playback speed - and with it the pitch - sweeps under your hand. R4 is one of the little surface-mount resistors along the top edge of the board, so the same blob-of-fresh-solder trick from Step 1 lifts it straight off.

Nearly every guide says 100K and stops there. It works - it is also the most timid of the three options:

Pot value vs. sweep
Value	What you get	Worth knowing
100K	Modest, safe range	What almost every other guide lists
500K	Much wider sweep	My pick - the sweet spot for range without weirdness
1M	The really deep groans	Buyer beware: at the far end it plays so slow and so quiet that people assume the box is broken

That 1M caveat isn't a fault - it's the range simply running out past the point of being useful. If you're building one for somebody else, fit the 500K or warn them, otherwise the first thing you'll hear back is that it stopped working.

Darren Shaw's walkthrough of the R4 swap is mirrored here: Modifying an ISD1820 into a Lo-Fi Sampler.

A soldering iron tip pressed against the row of small surface-mount components along the top edge of the module, at the position marked R4, while the board is held steady in a helping-hands clip. — R4 is the surface-mount resistor at the end of the top row. Same fresh-solder trick as Step 1.

The top edge of the module after removal, with a visible gap in the row of surface-mount parts where the R4 resistor used to sit. — R4 gone. Those two pads are where the pot picks up.

A bare potentiometer standing on the bench, seen from the front, with its splined shaft, threaded bushing and three solder lugs along the bottom edge. — The pot before it goes anywhere near the board.

Needle-nose pliers gripping the small anti-rotation tab on the face of the potentiometer to snap it off. The body of the pot is printed B500K. — Snap the anti-rotation tab off unless your enclosure has the matching hole. Note the body: **B500K**.

Close-up of the potentiometer with two wires soldered on, a yellow lead on one outer lug and a grey lead on the centre wiper lug, the third lug left bare. — Two wires: one outer lug and the wiper. The third lug stays empty.

The whole assembly laid out on the bench: the module with its yellow and grey pitch-pot leads running to the potentiometer at the bottom, the electret mic and speaker on green and yellow leads, and the lithium cell alongside. — Pitch pot wired in, everything else still connected — ready to sweep.

Other directions

Put it in something. Same chip is the heart of Loopy Louie (in a Gooey Louie head) and Bent Brooks (in a decorated skull).
Line out. Tap one SP1 pad through a coupling cap and a level pot for a jack, so the box can feed a pedal chain instead of only its own speaker.

All together

The complete build spread across the workbench: the ISD1820 module clipped in helping hands with its J1 header labels visible, two red toggle switches wired in, the lithium cell, the relocated electret microphone, the large speaker, and the pitch potentiometer, all connected with coloured hookup wire. — Everything wired: two toggles, two buttons, mic, speaker, pitch pot and cell.

Written by Onion Madder. Feed-through pin behaviour per the ISD1820 datasheet (module user guide); the four-mode panel wiring is worked out from my own builds. Build it, bend it, make it yours.