Introduction: "Dark Detector" Using a 555 Timer IC

About: I am currently a middle school student. I am primarily interested in the realms of computers and electronics, but I also enjoy playing the piano.

Well, this is my first instructable (yay) so, here it goes!

This is a Dark Detector circuit that utilizes 1) the astable ocillator that you can make with a 555 to drive a piezo and 2) the reset threshold of the chip.

Credits to Tony van Roon for the circuit diagram.

For Halloween:
I'm planning to either
1) combine it with a strobe light so with each "off" cycle, it screeches or
2) Put it over the doorbell (so it can be used seasonally too! :  ] )

Step 1: Step One: Gather the Parts


555 Timer IC (I used a Texas Instruments NE555P, you can use pretty much any other brand)
A breadboard (doesn't have to be big)
Some wire; bare or stripped (you can use staples if they are thin enough)
One 1 megaohm resistor
One 100K resistor
One 100 ohm resistor (a 100 ohm potentiometer is better but optional)
One 1000 picofarad capacitor (in nanofarads: 1 nF in microfarads: .001 uF)
A Cds cell
A Piezo siren/buzzer

The resistor values can be experimented with but I reccomend these values for the most sucess.
The piezo can also be a speaker.
The capacitor value should probably stay the same.
You can try different Cds cells.

A great place to find components is the Electronic Goldmine:

Step 2: Step Two: Put the Timer In

Stick the timer in the breadboard so that the notch and/or circle is facing to the left.

Step 3: Step Three: Stick the Negative and Positive Rails In.

If you haven't worked with circuits before, here comes the (semi) tricky part.
Connect the 1st pin with the negative rail.
Then, connect the 8th pin with the positive rail.

If you need help with the pinouts, see the Pin Diagram picture.

Step 4: Step Four: Connect Pins 2 and 6...

...with a piece of wire. Again, if you need help identifying the pins, consult the pin chart.

Step 5: Step Five: Stick (yes Stick) the 100k Resistor In

Connect the 4th and 8th pin with it.

Step 6: Step 6: Put (not Stick) the 1 Megaohm Resistor in It's Place

It's place would be connecting the 2nd pin to the 3rd pin.

Step 7: Step Seven: Stick the Capacitor In

Put it in pins one and two. If you have an electrolytic capacitor, make sure that the polarity is correct.

Step 8: Step Eight: Put the Cds Cell In

Make sure that it connects pin 1 and pin 4.

Step 9: Step Nine: the Hard Part (for Some)

Well, you've gotten this far. So, pat yourself on the back and get ready.

Look carefully at the picture. If that doesn't help, read below

_The Hard Part_
Take a piece of wire long enough to span about 5 to 6 breaboard-cells (if anyone knows the term, please tell me). Stick it to the 3rd pin. Stick the other side to an adjacent area in the board.

Now, take the piezo's postive lead and stick it in the adjacent cubicle. Stick the negative wire into the negative rail. Now, put the potentiometer in. the first lead goes into the into the same row as the wire. The middle lead goes into the column with the positive piezo.

Now, connect a fresh nine volt battery to the correct rails.

Thats it! You're done.

Step 10: Troubleshooting, Shooting Trouble, Et Omnis


If you have any problems, look over all of the steps.

Make sure that you put the wires into the correct pins-rows. When I first made this circuit, I put the Cds cell into pin 3 and pin 1 when it was actually pin 4 and pin 1.

Another problem could be that some of your wire/leads are touching/shorting. Make sure that the only thing the leads touch are the breadboard cubicles.

Maybe your components are broken. The 555 is very sensitive to static electricity.

Also, USE A NINE VOLT BATTERY! Alkalines are the best but rechargables will work. I used a power supply.

If you've tried the above and still can't get the circuit to work, drop me a comment and I'll see what I can do.


I hope you've enjoyed creating a Dark Detector. Some possible next steps are:

Puting it in an altoids tin (of course!)
Puting it in a McDonalds apple pie box (what I'm going to do)
Experimenting with different reisistor values.