Tulley Family Tinkering Lab Website

555 timer circuit success!

by on Mar.10, 2010, under science fair

Robbie and I went to bed last night thinking that his first 555 timing chip circuit did not work. The relay was either not firing at all, or firing in rapid succession. I realized that we probably needed a diode in the circuit, and researching it showed this to be the case. At that point, nothing worked at all — the voltage coming into the relay’s coil was too small to trigger the relay.

Overnight, one of my online friends pointed out that the diode needed to be connected in parallel to the relay’s coil (we had it in serial), and I realized that a transistor was needed to overcome the too-small voltage problem.

We needed this circuit: http://www.kpsec.freeuk.com/components/relay.htm#protect

After adding in the transistor and putting the diode in correctly this time, it worked! After the trigger came in we heard, “click”….. (2 second pause)… “click”!

555 circuit #1

555 circuit #1

555 circuit #2

555 circuit #2

555 circuit #3

555 circuit #3

Do you notice in these last two pictures the capacitance problem we had? I thought i had the correct capacitors, but we didn’t. Luckily Robbie’s Digital recording lab had one correct capacitor, and 10(!) other capacitors in series gave us the value we needed for the “control voltage” capacitance on the 555(really 556) chip.

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