The power supply varies, and the circuit must operate on under 10uA of current (not counting charging the cap). It triggers an SCR every 10-30 seconds as long as the power supply is above 1. 8VDC, and must operate across a range of 1. 8 and 7. 0 VDC. The timing isn`t critical - around 10-30 second intervals to trigger the SCR is fine (a short positiv
e pulse). The lower the voltage, the longer the time interval is fine. The kicker is the low current requirement (10uA or less), low voltage requirement (1. 8V) and, as always, low cost (ie, replacing a 10 cent PUT with a 30 cent microcontroller would not be ideal).