To diagnose malfunctions of digital TVs, set-top boxes and similar equipment, you usually do not need high-precision diagnostic instruments. In most cases, a logic tester is sufficient to determine the level state of the circuit being called. Since if mosfets or processors fail, they most often go into a short-circuited state, and most rarely into a break, but not into an average or floating state.
Now you will see how to make a penny tester, with which you can instantly determine the level: low (“minus” source), high (“plus” source), variable, uncertain or open.
Will need
- Disposable syringe.
- 3 LED different colors of glow - http://alii.pub/5lag4f
- 2 resistors 1.5 k Ohm - http://alii.pub/5h6ouv
- 2 alligator clips - http://alii.pub/5mfeqh
- Heat-shrink tubing - http://alii.pub/5orpa8
Making a simple tester from a disposable syringe
For ease of use in the hand, a large 60 ml syringe was chosen.
Solder the wire to the needle and drill holes in the body for the wire itself and 3 LED.
Solder the wires to the contacts LEDs and insulate with heat shrink.
Lubricate them with glue and install them in the syringe body.
We fix the wire from the needle with a nylon tie.
We assemble the tester circuit:
Solder the resistors according to the diagram.
Solder the wires to the clamps.
We insulate all components with heat shrink.
We put all the twists into the body of the syringe and fill them with hot glue.
The tester is ready for use.
Working with the tester
We connect the clamps to the power supply of the device that needs to be diagnosed. The total supply voltage in the logic circuit is 5 V.
Turn on the power of the device. The tester lights up blue Light-emitting diode, indicating an undefined level, since the needle probe is not connected anywhere.
We test the device. If you touch the ground with the dipstick, the red light will turn on. Light-emitting diode.
If to “+”, the green LED will light up.
If you touch the AC voltage of any frequency generator with the probe, both red and green LEDs will light up.
Such a simple test can quickly identify broken and faulty components.
This tester significantly saves time compared to the testing time with a conventional multimeter.
