the averaging functional block 302 upon making the decision decision functional block 304 output either an alarm a tamper or a secure signal to the rest of the system which will react in an appropriate manner by sounding an alarm signal for example if tamper or alarm are indicated referring again to fig3 the averaging functional block 302 and decision block 304 may consist of hardware and or software in the preferred embodiment a microprocessor operating in the control unit 201 is programmed to perform the averaging 302 and decision 304 functions as well as controlling the varying of the supervision voltage the reading of the adc 202 sounding the alarm etc in this embodiment the microprocessor could execute the following routine for example in order to perform the averaging and decision functions to correct for variations in the adc 202 readings ______________________________________begin averaging routineclear alarm tamper and reads countersdo loopincrement reads counterread adc values will be between 0 and 256 if adc value gt 134 and lt 162 or gt 94 and lt 122 increment alarm counterif value gt 162 or lt 94 increment tamper counterloop for 1 10th secondcheck which casecase alarm counter gt half readsindicate alarmcase tamper counter gt half readsindicate.