constant to momentary with a microrelay

But won't the cap only discharge immediately if the switched input goes to ground when turned off?Aaron

I knew something didn't look right when I uploaded the image. The following incorporates the NPN transistor that you can get from RS. It also includes the diode across the resistor so that the cap can discharge quicker. The zener diode is in the correct orientation. It looks odd but I inserted it there to basically force the capacitor to charge to nearly battery voltage before turning on the transistor. You may have to substitute a 1N5237B, which is a 8.2V zener, instead if it is not turning on consistently. You can probably get away without using it if you can't find one. Make sure you have the quenching diode across the relay coil, otherwise the transistor may get fried when the relay is turned off.