7555 timer circuit


Joined Nov 23, 2012
The 7555 should work the same whether 3v or 18V).

What voltage does your relay's coil need? What current?

Post a picture of your circuit (on the breadboard) - clear as possible.

Also, the 7555 can only handle 100 mA (in/out of pin 3) the 555 can work with 250 mA.


Joined Nov 30, 2010
41.2 megs to a 33 uf? You are asking that capacitor to be a very low leakage capacitor! (146 nanoamps is the current flowing through the resistors with a 9 volt supply.) This is why we use counter chips on a short interval 555 chip.


Joined Mar 14, 2008
I calculate 49.2 megs with 33μF to give a 27 minute time-constant or a timer period of over 29 minutes.

You will likely need very low leakage film capacitors (which are physically large) to achieve that period.


Joined Nov 30, 2010
Yeah, 5 x 8 megs plus 6 times 0.2 megs = me getting old.:p

The fact that I can remember what I was thinking when I blew that calculation (about 100 minutes ago) is amazing in itself!
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Joined Mar 24, 2008
The 7555 has incredibly weak drive at lower voltages, it is proportional to the supply voltage. At 3V it is about 2-20ma, while at 15V it is nearer to 100ma. You have to allow for this weak drive when using this circuit with a much higher gain transistor, among other things.

A conventional 555 does not have this problem.