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555 Timer Hacks: Cable Testers, Magnetic Stirrers, and Lego Grabbers Oh My!

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Step 2: 555 Timer IC Pins

Picture of 555 Timer IC Pins
The following are the pin outs for the 555 Timer IC (source:
http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php):

Pin 1 is ground. It is connected to the negative side of your battery or power along with any other components in your circuit connected to ground.

Pin 2 is the Trigger pin. It will be connected to ground and thus switches on pins 3 and 7.

Pin 3 is the Output pin. In this circuit it outputs a square wave signal that can be heard on a speaker.

Pin 4 is the Reset pin. It is not used in this circuit. See http://en.wikipedia.org/wiki/555_timer_IC or http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php for more info on this pin.

Pin 5 is the Control pin. It is not used in this circuit. See
http://en.wikipedia.org/wiki/555_timer_IC or http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php for more info on this pin.

Pin 6 is the Threshold pin. The 0.02uf capacitor C1 will charge up and when it reaches about 2/3 Vcc (voltage from the battery), this is detected by the Threshold pin. This will end the timing interval and send 0v to the Output pin 3 (switches it off).

Pin 7 is the Discharge pin. This pin is also switched off by the Threshold pin 6. When pin 7 is switched off it cuts the power to the 0.02uf capacitor C1 which causes it to discharge. Pin 7 also controls timing. Pin 7 is connected to the 50K ohm Variable Resistor resistor RV and the Photoresistor. As you move the slider on the Variable Resistor RV, it changes the amount of resistance in the circuit. This changes the timing of pin 7 and thus changes the pitch of the square wave heard on the speaker.

Pin 8 is connected to the positive side of your battery or power along with any other components in your circuit connected to positive.
 
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