PCB Tester. Start Up Project 2

After assembling the circuit on the Etched PCB or Perf board, we need a PCB tester to troubleshoot the problems if any. We can test the points on the PCB using a Multimeter to confirm whether current is arriving or not. But this handy tester will give an Audio visual indication. It beeps and lights LED when there is current flow in the test points. It can also be used as a Battery tester. If more than 2V is present in the battery, buzzer beeps and LED lights. If the battery is flat, the tester remains silent.It is a good tool, so learn to build this as your start up project.

You need the following components
1. 1K Resistor – 2Nos
2. Transistor BC 547 -1
3. Buzzer – 1
4. 9V Battery snap – 1
5. Alligator clip – 1
6. 9V Battery – 1

Component description
1K Resistor
As you know, a Resistor drops current entering into a particular section of the circuit based on its value. Here we need a 1K resistor. You can easily identify it using the colour code printed on it. First observe the Third colour. It is the Multiplier band. By noting the third colour we can assess the range of the resistor value. For example if the third colour is
Gold – 1-9.9 Ohms
Black – 10- 99 Ohms
Brown – 100 – 999 Ohms
Red – 1K – 9.9K
Orange – 10K – 99K
Yellow – 100K – 999 K
Green – 1M – 9.9 M
Blue – 10M
Here the colour code of 1K resistor is Brown – Black – Red
Brown is 1, Black is 0 and Red is the Multiplier. So 1+0 + 00 = 1000 Ohms or 1 Kilo Ohm
Transistor BC 547
It is an NPN transistor which is the general purpose one used in switching applications. It has three leads- Collector, Base and Emitter. When the base gets a biasing voltage, current flows from the collector to emitter. Thus it acts as a switch. We can easily identify the pins of BC 547. Hold the Flat side on which the number printed towards you. Then count the pins. First pin is collector, second one base and the third pin emitter. So the arrangement is C-B-E. Its complementary PNP transistor is BC 557 and its pin assignment is E-B-C.

BC 547 DATA SHEET

BC 557 DATA SHEET

Buzzer
It is the Piezoelectric buzzer that has a circular plate with Piezo electric material. There is an oscillator circuit with resistors, capacitor and a transistor. When we connect the buzzer to battery, the circuit inside oscillates and the Piezo disc vibrates. These vibrations generate sound. The PCB mounting type Piezo buzzer works off 3-12 V DC.
9V Battery Snap
This is used to take power from the 9V battery. It has two clips to connect with the battery and positive and negative leads.
Alligator clip
Small clip to hold wires in place
9Volt Battery
It is the general purpose PP3 battery used in Projects. It is generally 500 mA rating.

How it works?
The Negative of Piezo buzzer is connected to the Collector of BC547 and  its positive to the positive of battery. The base of BC547 has the 1K resistor to restrict the base current. The other end of 1K resistor is free and acts as a Probe to touch with the PCB under test. The emitter of BC 547 is connected to the negative of battery. The Alligator clip is connected to the negative of battery using wire. It is used to connect with the negative of the PCB under test. So this is the testing procedure.

Connect the Alligator clip to the negative of the PCB under test and connect power to the PCB. Touch the positive rail of PCB with the free end (probe) of resistor. Buzzer sounds to indicate the presence of power in the PCB. Similarly test each points of the PCB to confirm the presence of power. Remember, the negative sides will not give any indication.

Transistor BC 547 is doing the trick. When we touch the probe to the test point, if power is available, base of the transistor will be biased and it then conducts. The buzzer then gets a current path through the collector-emitter of the conducting transistor.

Testing the Bread board assembly power using the PCB tester.

Testing 9V Battery. It is not flat

Testing battery. It is flat

See the step wise method to make the PCB Tester