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Three-State Continuity Tester
When the resistance is high, green diode D3 lights; when it is of medium value, yellow diode D2 lights, and when it is low, red diode D1 lights. The levels at which the diodes light is set with P1, but note that in any case the minimum value depends on the p.d. across R2. It is possible to reduce the value of the p.d. to enable lower transfer resistances to be detected, but this would mean an increase in the test current through R2. With values as specified, the circuit in its quiescent state draws a current of about 17 mA, but in operation each LED adds about 10 mA to this. The LM324 (IC1) may be operated from a single supply line: R1 prevents the voltage at the input from reaching the level of the supply line (which is not permissible). The supply voltage may be 5–18 V. The LEDs are driven directly by the inverters in the 4049 (IC2), which can switch currents of up to 20 mA to earth.