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Additions to my interval circuit

A project log for Landy Wiper Control

Sick of having 2 speeds for my windscreen wipers. Nothing a 555 timer can't fix!

tron9000Tron9000 07/26/2017 at 20:190 Comments

Had a quick tinker with this project last night.

With the new update for the wiring schematic meant that the output of my Interval circuit (see above).

The relay I'm controlling requires it to be driven by 12V instead of being pulled down to 0V.

Considering that the output of the timer is low for <1 second and high for the set interval, I need a PNP or P-channel MOSFET to drive the relay from the 555 timer output.

The chosen device to do this: 2n3906 - cheap, readily available, and up to the job.

After bread-boarding up the circuit, I came up with this schematic:

It works well but there is a strange anomaly where upon first power up and the reset circuit (consisting of R7, R8, C3 & C4) forces the output low, there was a delay between the output of the 555 going low and the test relay I was using energising. After that it worked as expected.

As you can see I also connected one of the positions of the rotary switch (S2) directly to pins 2 & 6 of the 555 timer. This is instead of bypassing the 555 altogether and connecting power directly to the relay. The reason for this is that according to the 555 datasheet:

So with 12V at these pins, both conditions are satisfied and the output will be low constantly. However this will need testing first.

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