Test continuity with an LED

ON Semiconductor BC549

Raju R Baddi


You sometimes need to know whether a resistance exceeds a preset limit. The continuity tester in Figure 1 lets you determine that fact for resistances of 0.5 Ω to 10 kΩ. The heart of the circuit is the transistor pair comprising Q1 and Q2, whose emitters draw current from a single source, RE. Insert the circuit under test, RCY, between Point A and Point B. To set the limit, use a known resistance for RCY and set the trimming potentiometer until the LED begins to light.

This continuity tester lets you know whether a resistance exceeds a preset limit.
Figure 1. This continuity tester lets you know whether a resistance exceeds a preset limit.

The current through RE divides between Q1 and Q2 in proportions based on the resistances of the two loops. The circuit lets you set the low limits to values as low as 0.5 Ω because the emitter current in Q2 can change rapidly with small changes in its VBE (base-to-emitter voltage). The remaining current originating in RE goes through the emitter of Q1, whose collector then suffers voltage changes on the order of approximately 100 mV because most of a transistor’s emitter current flows to its collector.

At extremely low limits, a large change in emitter current can easily accommodate the drop in voltage across RCY in LOOP 2. The extra current goes through LOOP 1. At the critical value of RCY, LOOP 1 conducts a much higher current than LOOP 2, which again means a much smaller VBE change for Q2.

Here [1] you can download an appendix that provides a detailed analysis of the circuit’s dc performance.

When RCY is an open circuit or has a resistance above the set limit, a larger portion of the current through RE flows to the emitter of Q1, which produces a voltage across R3. That voltage is close to the voltage at the emitter of Q3. Thus, Q3 doesn’t have sufficient VBE to turn on. In turn, Q4 is off, and the LED doesn’t illuminate.

When the resistance of RCY is under the set limit, Q2 begins to draw its share of current from RE. This step reduces the current through the collector of Q1, and the voltage drop across R3 also decreases. The difference in voltages between the collector of Q1 and the emitter of Q3 exceeds VBE. Q3 then conducts, turns on Q4, and lights the LED.

The tester’s quiescent current is 10 mA, making the tester suitable for a bench instrument. If you need battery power, such as a 3.6 V nickel-cadmium or lithium-ion battery, however, you can reduce the LED’s series resistance by less than 47 Ω and change Q3’s emitter voltage. (See [1]).

Use two variable potentiometers in series whose values – 1 kΩ and 100 Ω, for example – differ by an order of magnitude. This approach allows you to make precise limit adjustments at lower limits.

The values in parentheses in Figure 1 are substitute values. You can substitute five 1N4148 diodes for the 3.2 V zener diode. Both arrangements perform well. The LED may go a bit dim toward the low limit, approximately 0.5 Ω, so use one with a transparent lens.


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