Here’s my entry in the 555 design contest. I wanted to make a very minimal circuit for the contest and only use stuff I had laying around. This is what I came up with.
After being in the dark for a few minutes, the piezo buzzer starts up and emits a really irritating tone, but it turns off almost as soon as light hits it. Hide this in a lamp or light fixture before somebody goes to bed!
Here’s a video description:
I originally built this circuit on a breadboard, but wanted something nicer for the contest (and easier to hide) so I made a quick toner transfer PCB. Not my best work: the ground plane is broken into two pieces, hence the black jumper.
|Yeah, the cap is big.|
|When I added my name on the board, I broke the ground plane into two parts.|
How it works:
R1, R2, and C3 and the 555 form an astable multivibrator that drives the piezo buzzer (I used JP2 for the footprint to save time). Basically C3 chargers through R1 and R2, but once the voltage gets to about 2/3 Vin, the flop flop in the circuit is reset and that discharges C3 through R2. Once the voltage drops to about 1/3 Vin, the flip flop is set and the cap starts charging again.
Meanwhile, pin 4 on a 555 is a master reset pin and pulling it low will hold the flip flop low. So it needs to be high for the circuit to oscillate. C1 is the big electrolytic cap (2200 uF). It charges slowly through the voltage divider formed by R3 (20 Meg in my example) and the CdS cell (about 1-2 Meg in the dark). Once C1 charges to about 0.6 V, the buzzer starts. It needs to be dark for this to happen. When light hits the CdS cell, the resistance drops to about 100K so C1 rapidly discharges and turns the buzzer off.
So long as Vin X R5/(R3+R5) > 0.6 the circuit will work as described. Increase R3 and make C1 bigger for a longer turn-on delay. Ideally, you want somebody to go to sleep before the buzzer turns on.
A lot of circuits you find online use pin 4 to make a light or dark activated buzzer, but I could not find any existing circuits that use this configuration to generate different turn on and turn off time constants.
|Here is the schematic in Eagle. I’m not a fan of the 555 part layout in the library.|