Audible Logic Probe using the 555 Timer

Feb 012023

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Why to use an audible logic probe?

Many logic probes use LEDs to visually indicate the logic state of the point under test. This causes the user to have to see the point under test and the LED at the same time.

This could cause possible measurement errors and could even damage the equipment under test due to inadvertent movements of the logic probe.

The audible logic probe shown here solves this problem by using an audible prompt. A high frequency beep when the sensed voltage level is high, and a low frequency beep when the sensed voltage level is low. When there is a train of pulses, the signal that is heard is intermittent high frequency and low frequency.

This audible logic probe can be used with both TTL and MOS circuits. The circuit’s operating voltage ranges from 4 to 15 V, depending on the circuit being tested. Current consumption is approximately 10mA on a 5V circuit and 35mA on a 15V powered circuit.

How does the audible logic probe with 555 circuit works?

Two comparators, from a chip with four, are used to sense the high and low voltage levels at the input. A voltage divider establishes the necessary voltage references. (R4, R5, R6)

1 – In the low level detector IC1A. When the voltage at the test probe at pin 5 of IC1 is greater than the reference voltage at pin 4, the output of this comparator goes high. This, forward biases the diode D2.

When this happens, resistors R7, R9 and the 555 IC produce an approximate 3.5khz tone. This tone is the logic high level signal.

2 – In the low level detector IC1B. When the voltage at pin 6 is less than the reference voltage at pin 7, the output at pin 1 of the LM339 IC goes high level. This, forward biases the diode D3.

Diode D3, together with resistors R8 and R10, causes the 555 timer, which is configured as a stable multivibrator, to start its operation.

Since this resistor combination is larger than resistors R7 and R9, the oscillation frequency is approximately 300 hertz. This signal will be the logic low level tone. The voltage divider network R2 and R3 maintain the quiescent state at approximately 1 V on the test probe when it is not connected to any signal. This way, no comparator will cause a tone to be generated on the 555.

Input voltage levels

In TTL technology, a low level is usually specified at 0.8 V and a high level at 2 V.
In MOS technology, the voltage levels are generally specified as 1.5 V for low level and 3.5 V for high level.

Audible logic probe operation.

As the energy of our audible test probe is obtained from the circuit under test, the ground clip is connected to the circuit ground and the positive clip is connected to the positive of the circuit. If the probe is not being used, no sound will be heard from the speaker.

When the test probe is connected to the positive of the circuit, a sound with a frequency of 3500 hertz approximately will be heard from the speaker.
When the probe is connected to ground, a sound with frequency 300 hertz approximately will be heard from the speaker.

If the probe senses a train of pulses, it will cause an audible signal of approximately 10khz.

LM339 – Internal Configuration and Pinout