🚀 go-pugleaf

Andrew Reynhout was looking for a circuit that activates a relay for
one second on a rising edge on a logic input.

In a followup I suggested using a 555 timer as a one-shot driving the
relay. Besides the fact that the 555 is started by making the trigger
input low (this is solved by adding an invertor) there is a slight
(nonobvious) problem when the trigger signal stays active longer than
the output pulse of the timer. The application notes almost forget
to mention that this situation should be avoided. You will have to
ensure that the logic input signal will never stay on longer than the
output pulse.

If it is not possible to ensure that the input signal is low at the
end of the 1 second period, you'll have to add some logic that switches
off the trigger signal of the 555 after it has been started until the
logic input signal has gone low again.

The following circuit should perform that function:
                                        ___
Input >---+-----------+----------------|   |
          |           |                | & |o---> Timer trigger input
          |           |             ---|___|          (active low)
          |           |    ___     |
          |            ---|   |    |
          |               | & |o---+
          |        -------|___|    |
          |       |                |
          |       |    ------------
          |       |   |
          |        ----------------
          |           |    ___     |
          |      ___   ---|   |    |
           -----|   |     | & |o---
                | & |o----|___|
Timer output >--|___|

The 555 timer must be wired as follows:
Pin 1: GND
Pin 2: Trigger, active low, connect to output of circuit above, or to
       inverted logic input signal
Pin 3: Output (also drives the relay; don't forget the diode across the
       coil of the relay)
Pin 4: Reset active low, connect to VCC (max current is 200 mA)
Pin 5: 10 nF to GND
Pin 6: Threshold, connect to RC network
Pin 7: Discharge, connect to Pin 6 (and thereby to the RC network)
Pin 8: VCC

The RC network consists of a 1MOhm resistor to VCC and a capacitor of
1 uF to GND. (The one-shot time of this circuit is 1.1 * R * C so this
yields 1.1 second.)

Disclaimer: I have not tested this.

Success!,

Peter Knoppers - knop@duteca.et.tudelft.nl