Introduction: Logic Probe With Pulse Detection

The TWO TRANSISTOR LOGC PROBE introduced by jazzzzz

is simple - but not stupid - it works very well determining the logic level of TTL and CMOS.
A major problem in digital circuit testing is detecting pulses and glitches. The TWO TRANSISTOR LOGC PROBE

  • fails at frequencies above 500kHz and

  • a 1ms glitch can not be seen.

Step 1: Detecting Pulses

A circuit consisting of a MOSFET, two diodes, two capacitors a LED and a resister solves this Problem.

If the probe detects a pulse the LED will glow for 1 second. The good news: it will detect single pulse down to 100ns.

Step 2: How It Works

A raising edge of a pulse loads the two capacitors via C1 - D3 - C2.
The voltage at C2 raises much more than at C1. The voltage at C2 is the gate voltage of the MOSFET. The MOSFET switches on and the LED lights up.

The capacitor C1 is discharged by the leakage current of the diode D3. The MOSFET switches off when C2 is discharged.

A falling edge of the input signal discharges C1 via diode D2.

The timing is not very well determined because it depends on the diode D3. It may be necessary to change the capacitors: no C2 and/or C1 = 100pF. A resistor of 20MΩ could solve the problem but it is not easy to purchase.

Step 3: Testing the Pulse Detector on a Bread Board.

The image show the pulse detector on the right.

The LED is almost on. That is because the circuit is very sensitive. We have to put a resistor between the input and ground.

Connecting the input to the positive source, lights the LED for one second. This time depends on the capacitor C2. The circuit still works without C2. The the LED lights up shorter. The cause is the gate capacitance of the MOSFET.

If there are pulses at the input the LED lights all the time. At a frequency below 1Hz it flashes.

It still lights at a 20Mhz.

The 74HC00 on the left side generates very short pulses.

Step 4: Testing Very Short Pulse

We need a circuit generating very short pulses.

We use two NAND gates of a 74HC00. The gate IC2A inverts the input T. The second gate is not ((not T) and T).
That is always 1. The gate IC2A needs some time to generate its result. If T was 0 and changes to 1 then IC2A is a short time still 1 and the gate IC2B gets for a short time a 1 on both inputs. IC2B generates a short 0 spike. This spike is the range of 10ns.

A professional spike detector will detect a spike of 10ns but ours. We can stretch the spike using the capacitor C2=100pF at the output of IC2A. Then the spike is about 200ns.

Our spike detector detects spikes of 200ns.

Step 5: Improved Two Transistor Logic Probe

The jazzzzz logic probe

can be improved.

We insert one more resistor and a zener (D1).

The zener limits the voltage to 3.3V.
Then the LEDs is never dimming at voltages above 4V. The zener improves the detection of LOW.

U0 = Uz - Uled - Ube = 3.3V - 2.2V - 0.6V = 0.5V

This is in the range of 0.4V to 0.8V of TTL Low. The voltage at the green LED is 2.2V.

The HIGH level depends on the voltage of the red LED and is

U1 = Uled + Ube = 1.8V + 0.6V = 2.4V.

This is the TTL High level.

The zener of 3.3V is important. A ZF3.3, BZX79-C3V3, 1N5226B or 1N4728A ca be used.

Step 6: Putting It Together

If we put the pulse detector and the transistor logic probe together we get a useful logic probe. The LED4 has not just inserted to protect LED3 against reverse polarity but to indicate this.

The layout of the logic probe is designed for BC337 and BC327. The flat side of the transistors is on the pc board. The 2N4401 and 2N4403 will work too but the pinning is reversed. Thus they have to be inserted with round side down.

The logic probe is build on a vero board and put in a transparent shrink tube.

Step 7: Results

The logic probe

  • is very cheep, only some cents
  • works at 3V to 12V
  • detects TTL and CMOS levels
    • Low @ 3.3V = 0.5V
    • Low @ 5.5V = 0.7V
    • High @ 3V to 12V = 2.2V
  • is protected against reverse voltage up to 12V and
  • input voltage -12V to +12V

  • detects
    • Low/High (green/red LED) up to 100kHz @ 3.3V and 500kHz @ 5V

    • single pulses down to 200ns
    • frequencies up to 20MHz ( blue LED )
  • draws
    • a supply current less than 7mA @ 5V
    • an input current less than 25µA
  • has an input capacity of about 150pF.

Step 8: More Information

You can get more information (in german) on logic probes