Transistor Tester Circuit
TransistorTester.gif

Link to original article. My articles likely has errors. Things I have wrong or don't understand yet.

Overview

The 555 operates at 2Hz (two times per second). Output pin 3 drives1 the circuit high with a positive voltage of 6v, then off2.

The other end of the circuit is connected to a voltage divider with the mid-point at approx 4.5v. This allows the red and green LEDs to alternately flash when no transistor is connected to the tester.3

With the Transistor Connected

If the transistor is good, it will produce a short across the LED pair when the voltage is in one direction and only one LED will flash.4

If the transistor is open, both LEDs will flash.5

And if the transistor is shorted, neither LED will flash.6


Footnotes
1. Pins 3 voltage out is always 2/3 of the rail. In this case, about 6 volts.
2. Goes HIGH (about 2/3 of the rail) and LOW (about 1/3 of rail).
3. Pin 3 alternates between source and sink. Reminder, it it is 2/3 of rail out (as a source) and 1/2 of rail as an input (sink).
4. The (1N4148) diodes alternate depending on whether it is an NPN or PNP transistor present.
5. Open is the same as missing?
6. Shorted is the same as a soild wire being in place instead of the transistor.

Timing Calculations


Using the 555 Timer Calculator we can see the LED flashes about two (2) times per second.

  • 0.229 Seconds = Time High (secs) = 0.693 * (R1 + R2) * C1
  • 0.229 Seconds = Time Low (secs) = 0.693 * R2 * C1
  • 2.186 Hz = Frequency = 1.44 / ((R1 + R2 + R2) * C1)
  • 50.000% = Duty Cycle Percentage = (Th / (Th + Tl)) * 100


Note Use R1 = 1 and R2 = 33000.

Pin States

  • Pin 8 gets the power.
  • Pin 4 is pulled up. Setting it low resets the timer. Letting the pin float (unconnect) may cause random resets.
  • Pin 7 floats
  • Pin 2 (trigger) & Pin 6 (threshold) monitor the timing capacitor. In effect this sets the duty cycle.
  • Pin 1 tied ground.
  • Pin 5 floats.
  • Pin 3 outputs about 2/3 of VCC (around 6V). The in-series current limiting resistor will result in 6 mA delivered to the GREEN LED.
    • For the GREEN LED, about 150 Ω for a 3V/20mA LED and 200 Ω for a 2V/20mA LED will allow better illumination.
    • For the RED LED, about 4.5v is delivered. About 82 Ω for a 3V/20mA LED and 150 Ω for a 2V/20mA LED will allow better illumination.

Parts List

  • 1 - 33kΩresistor
  • 4 - 220Ωresistor
  • 1 - 10µF polarized capacitor
  • 2 - 1N4148 diodes
  • 1 - red LED
  • 1 - green LED
  • 1 - 9-v power (battery or power supply)

Reference(s)

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