Making a Simple Temperature Indicator Circuit

The temperature of a heat-sink can be measured with a wet finger: if it sizzles, the temperature is too high.

The circuit of a simple temperature indicator in the shown diagram is an alternative method of checking that does not cause blisters: A green ’safe’ LED lights as long as the temperature of the heat-sink does not exceed 500 Centigrade, an orange 'caution' LED for temperatures of 500..750 Centigrade and a red ’danger’ LED for temperatures above 75° Centigrade.

The circuit is simplicity in itself, two special zener diodes, D1 and D2, are connected in series to ensure an accurate zener voltage of 5.96V at 25° Centigrade. The zener voltage will rise by 20 mV for each degree Centigrade rise in temperature. The voltage level corresponding to the temperature of the heat-sink is compared with two reference voltages by IC1 and lC2.

When the temperature reaches 50° Centigrade the output of lC2 goes high so T3 conducts and causes D4 to light and at the same time D5 is extinguished by T4. At or above 75° Centigrade the output of lC1 is high and T2 and T3 then conduct to make D3 light and D4 extinguish. Under normal conditions, that is, considering a heat-sink of sufficient cooling area, a temperature of 75° Centigrade will never be reached.

Parts List for the above Simple Temperature Indicator Circuit

R1 = 22k

R2 = 5k6

R3, R12 = 820E

R4 = 220E

R5 = 180E

R6 = 470E

R7 = 4k7

R8, R9, R10, R11 = 15k

Semiconductors

D1, D2 = LM335 (National Semiconductor)

D3 = LED red 5mm

D4 = LED orange 5mm

D5 = LED green 5mm

T1, T2, T3, T4 = BC547B

IC1, IC2 = 3140