Based on only two ICs, this circuit can perform three test functions as selected by switch S1.
The circuit is based on a clock generator, IC1a-1c• that provides a rectangular signal at a frequency of 3 Hz.
With S1 in position A, T1 applies the clock signal to the probe. This buffer stage can deliver a current of up to 100 mA via limiting resistor R7. In practice, this means that the probe can inject the 3 Hz signal into any digital circuit, thereby. overruling all logic outputs. Only short circuits can cause the signal to disappear and the probe can, therefore, quickly detect them. This is, however, a rough and ready test, because it means that the tester short-circuits all outputs connected to the track that is being examined, whereas manufacturers allow this to happen to only one output at a time.
With S1 in position B., the tester can detect logic levels. When the probe is not connected to anything, the clock is present at the input of ICid, so that DI flickers in a 3 Hz rhythm. When a logic level is applied to the probe, it suppresses the clock: if the level is low, the LED goes out: if the level is high, the LED lights continuously.
The switching thresholds of IC] lie at about 1/3 and 2/3 of the supply voltage, which is suitable for the CMOS 4000 series and the HCT family. Standard TTL, LS, and HCT ICs may be used, but the thresholds are then not optimum.
The tester can work with input signals at frequencies of up to 30-40 MHz.
With SI in position C., the input signal is applied to the clock input of shift register IC2. After eight clock pulses at the probe, the high level that is constantly present at inputs. A and B of IC2 will have arrived at the OH output so that D2 lights. Since IC2 is constantly reset by the 3 Hz signal. the LED will flicker as long as there is a-clock
at the probe, provided that its frequency is higher than 25 MHz.