Crystal tester circuit using BC107

You cannot use a multimeter to check a crystal. You should have a this circuit, because it can check a crystal 100Khz to 900Mhz, easily to make and cheap.The crystal is a key equipment used to generate a radio frequency.

– Some circuits use a clock signal controller. For example, The digital circuits of computer system. And widely used in quartz watches, both pin and the numbers family.

– This crystal determines the accuracy frequency and widely used in the present.
-External shape of the crystal is a cans Flat or circular the side of the tank out type characters to specific frequencies such as 5.7143, 3.597, 2MHz etc.

Sometimes, the circuit of us do not work. We can check other devices by the meter. But the crystal will know how good or bad. Important point, the Crystal is expensive and time consuming to buy new. To buy a lot of inventory it is not worth, because we not often to use it. So we had better create a crystal tester circuit.

How it works.
Beginning, we look it work before, in Figure below.
-The transistor Q1 (BC107) is the key to the oscillated circuit. By will create frequency, when the crystal put on socket to the base pin (B) of Q1.


-When the switch S1 is pressed to the circuit, the current from the battery (B1-9 volt) to flow into circuit, the oscillated signal from emitter pin (E) of Q1 pass through to C4.

The oscillator signal will be changing all the time, need to smooth with D1 and D2. Then, the signal is passed to the C5, which helps smooth up. At C5 has voltage across it, is a positive voltage to trigger a bias current to the Q2 works, as a result the LED1 lights. The R3 limits the current through the LED1 to a value not exceeding 20 mA. If more than this, LED lights too, and short lived to LED1.

The above principle see that LED1 is illuminated when the transistor Q1 works. The Q1 will work, and build frequency, it requires that the crystal quality. to use this principle, we can easily measure the crystal.

The creation and applied.
This project use equipment less. Therefore can be soldered onto the perforated board, easy to save money and time.

When create a circuit is complete, it’s the test circuit. First, find a good crystal to test in the circuit. Then press S1 to see LED1 brightness. But if it is shut down showing that the circuit may be wrong. You may have forgotten the battery or alternating polarity LED1 or other.

The parts of circuit
Q1,Q2__________ BC107 npn transistor = 2 pcs
D1,D2__________ DRL=1N4148 75V 150mA Diodes = 2 pcs
R1_____________ 27K______ 1/4W Resistor = 1 pcs
R2_____________ 1K______ 1/4W Resistor = 1 pcs
R3_____________ 560 ohm__ 1/4W Resistor = 1 pcs
C1_____________ 0.1uF 50V Polyester Capacitor = 1 pcs
C2,C4__________ 330pF 50V Polyester Capacitor = 2 pcs.
C5_____________0.0047uF 63V Polyester Capacitor = 1 pcs.
LED1___________ any color as you like.
Finally, we have the electronics tool next one on our desk. it can be used to check the crystal used in radio transmitters, radio control, the digital computer clock circuit and so on.

NOTE: This circuit not have PCB layout so you can see the circuit below with PCB layout. Which is similar structure of the the circuit. but detail of components slightly different. The hope is to build this project even easier.

Figure 2 the crystal tester circuits using BF199 and BC548 same circuit above.

Figure 3 The PCB layout and The components layout of the Crytal tester projects.


  1. Rickey August 27, 2012 Reply
  2. iumez November 5, 2013 Reply
  3. Earle August 4, 2014 Reply

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