Six Halloween circuits from the spring terminal
A candle that never sits still, eyes that blink the opposite way round, a lightning burst with a rattle, a door that groans itself shut, a tripwire that screams, and a ghost that remembers you crossed the room.
None of these need a part the kit does not already have. Every one of them is proved against the simulator before it goes in the drawer, which is why there are six and not eight: two of the ideas on the list turned out to be circuits that already exist under another name, and one of them was a touch plate, which the kit does not have.
Each link opens the recipe's page in the manual with its drawing. Press Wire it for me and the board does itself.
Turn on Night Shift from the Ladder first. The shop after closing, with only the parts carrying current lit, is the whole point.
1. A candle that never sits still
Two 555s, two 330 Ω, one yellow LED. One chip blinks about four fifths of a second, the other three times a second, and each drives the same LED through its own resistor. Both low and the LED is out; one high and it is half lit; both high and it is full. Because the two rates do not divide into each other the whole pattern only comes back round every five seconds, which is longer than anyone watches a candle.
Put the slide switch in the supply rail and you have a jack-o'-lantern with a lid.
2. Eyes in the bushes
One chip, one output, two red LEDs — and they blink opposite ways round, which is the bit worth understanding. The board's red LED hangs from +9 down to the output, so it lights when the chip pulls the output low. The drawer's red LED hangs from the output down to ground, so it lights when the chip pushes the output high. A 555's output drives both ways, which is how one pin runs two lamps that can never be lit together.
The upper eye never quite closes. There is about a volt left across it when the output is high, and that is an ember, not a bug.
3. Lightning, with a rattle
The trick here is where the lamp sits. It is not between an output and a rail — it is between two outputs, a slow chip's and a fast one's. So it can only light when the fast chip is high and the slow chip is low, and the fast chip's flicker gets switched on and off by the slow one all by itself: seven or eight flashes over three quarters of a second, then three quarters of a second of nothing. The speaker hangs across the same pair through 1 kΩ and rattles with them.
There is no white LED in the kit. In a dark room the red one reads as lightning anyway.
4. A door that groans itself shut
Hold the button to fill the 470 µF, then let go and stand back. The capacitor is now the only thing feeding the timing network, so every charge takes a little longer than the last and the pitch walks downward — 340 Hz, 300, 250, 170 — for about ten seconds. When the capacitor gets down to two thirds of the supply the chip can no longer reach its upper threshold, and the door is shut.
Compare it with experiment 22, where the pitch is yours to set with a knob. Here it is the capacitor's.
5. A tripwire that screams
This is experiment 18's dark alarm with something better on the end of it. The 100 kΩ and the light cell make a divider, the two transistors together have a gain of about ten thousand, and the relay pulls in when the cell goes dark. What the contact carries is not the sound: it is the 555's supply. No light, no relay, no 9 V on the chip, no scream — and the instant the beam breaks, the chip wakes up already oscillating at about 660 Hz.
Slide the light toward the moon, or turn the camera on and walk in front of it.
6. A ghost that remembers you
Two relays, and neither of them is clever. The first one is the tripwire's front end. Its contact gives the second relay a single push, and the second relay's own contact feeds its own coil — so once it is in, it holds itself in. The front end lets go the moment the light comes back and it makes no difference at all: the buzzer keeps going. The slide switch is the only way to shut it up.
That is one bit of memory made out of a hinge, and it is how every burglar alarm before the microprocessor knew you had been there.
And one you already have
The seventh is not new. Experiment 5 puts the light cell where a timing resistor was, so the pitch follows your hand: wave over the cell and it wails, cover it and it dies away. It has been on the bench since the first day. It just did not have a costume.
The Spooky Science Lab
Through October there is a mission laid over the six experiments these recipes are built from — the flasher, the light-controlled tone, the siren knob, the dark alarm, the latching relay and the pad beacon — told in the shop after closing, with a sticker at the end. It goes away on 1 November and comes back next year. Anything you earned is yours to keep.