555Timer

555 Timer Circuit

This circuit has endless uses. The circuit drawing was done by Scott Blake. The author needed the circuit to cycle a boat livewell pump off and on. The circuit uses the 555 timer connected as an astable multivibrator. The circuit is turned “on” when the OUT line of the 555 is in a high state, turning on the switching transistor Q1.

The “on” time for the circuit is given by the equation: Th=0.7(Ra+Rb)Ct
While the “off” time is given by the equation: Tl=0.7 Rb*Ct

In the illustrated circuit, Ra is synonymous to R2, Rb is synonymous to R1 and Ct is synonymous to C2. The values in the circuit give a theoretical duty cycle of about 7.1 seconds on and 3.6 seconds off. Experimental values of Ra=3K, Rb=390K and R3= 1K resulted in about 90 seconds on and 60 seconds off.

The 555 timer operation is explained at:
http://www.ee.ed.ac.uk/~kap/Hard/555/node2.html
The National Semiconductor datasheet can be found at:
http://www.national.com/pf/LM/LM555.html