Using a conventional logic probe, it is dif cult to determine what is happening at a point in a circuit above 50 Hz. The human eye isn’t able to perceive the change which is happening at such a fast speed—for instance, the ickering of a tubelight.
The audible logic probe described here helps to assess the logic state at a point in a circuit. The rate of change of state (frequency) is indicated by a tone. The audible probe has a large bandwidth ranging from DC to greater than 2 MHz. It uses a frequency-divider counter (CD4040), which divides the original signal frequency by 20 (1) to 212 (4096).
The circuit (shown in Fig. 1) is built around op-amp LM339 (IC1) and CMOS chips CD4069 (IC2) and CD4040 (IC3). CD4040 is a 12-stage ripple-carry binary counter/divider and oscillator. The tone is generated with the help of a piezobuzzer together with the divider counter. The piezobuzzer is driven by gates N3 and N5 of IC2, which are wired in anti-phase to each other. The probe can handle both TTLand CMOS-level supply.
The logic probe comprises the opamp (IC1) and gates N1 and N2 of IC2, which drive the dualcolour LED (LED1). Three 10- kilo- ohm resistors (R3, R4 and R5) in series provide the biasing for the two op-amps in IC1. High logic is set at two-third of the supply voltage, whereas low logic is set at onethird of the supply voltage.
When the voltage at the probe goes below one-third of the supply voltage, the output of upper opamp/comparator IC1(A) goes low. When the probe voltage goes above two-third of the supply voltage, the output of lower comparator IC1(B) goes low, thus lighting the respective LEDs. When the red LED glows, the logic is low or ‘0’. When the green LED glows, the logic is high or ‘1’.
When the probe is placed at a particular frequency junction, the received clock is divided by CD4040 (IC3) to give an audio tone through