Using a Remote Control to Make a Crystal Oscillator Frequency Tester

Quartz crystal oscillator is called "crystal oscillator" for short. It is widely used in electronic clocks, intercoms, single-chip microcomputers, color TV display line field oscillation frequency division circuits and all infrared remote controls. To judge whether a crystal oscillator is good or bad, we can only judge whether it is leaking or short-circuited or measure the capacitance between its two legs to roughly estimate its quality. However, it is impossible to measure its frequency deviation or failure to oscillate. Instruments for measuring crystal oscillator frequency are expensive, difficult to buy, and have low utilization rates.

In fact, you can make it yourself. The cost of materials is only more than 10 yuan (a color TV remote control and a small circuit board). After the materials are prepared, the modification can be completed in half an hour. In conjunction with a digital frequency meter, you can accurately measure the crystal frequency of the remote control and judge whether the remote control is good or bad.

The modification method is as follows: remove the back cover of the remote control and solder off the 455kHz crystal on the circuit board. Use a double-row 8-pin IC socket as a test clip, and only use a single row, 1~2, 3~4 pins are soldered together and soldered on the circuit board as a crystal test clip, the purpose is to facilitate the insertion of crystals of different sizes. Use 2 waste ballpoint pen copper heads as frequency meter test terminals and solder them together with the crystal test clip. Connect a 2-core shielded wire to the remote control crystal and solder it. Use the 3V power supply on the remote control to power the additional circuit board. Connect 5 wires (2 power supplies and 2 tests) to the remote control with the additional small circuit board. The other wire is connected to the remote control IC output pin (that is, the B pole of the NPN transistor on the remote control) through R2. The crystal to be tested is plugged into the test clip socket. Press the remote control button and the light-emitting diode will flash. If a digital frequency meter is connected, the remote control crystal frequency can be accurately measured.

If you aim other remote controls at the infrared photodiode button, the LED will flash, indicating that the remote control is good.

The IC model of the color TV remote control is Mitsubishi M50560-001P, and the crystal oscillator is connected to two 100pF capacitors. The frequency meter is a Victory brand VC920 pocket digital multimeter with a frequency measurement range of 0 to 20MHz. Experiments have proved that the notch filter and the three-pin crystal oscillator can start oscillation. The crystal oscillator below 1MHz can be accurately measured by the multimeter's frequency measurement function. The crystal oscillator greater than 1MHz will stop oscillating when measured by the multimeter. When the crystal is inserted into the remote control, D2 in the figure below will still glow. Color TV remote controls are similar and have the same working principle, as long as the crystal frequency is less than 7.15MHz. Other models of color TV remote controls can also be made in the same way. The circuit is shown in the figure below.

The circuit in the figure below is very simple. Two NPN transistors form a composite tube to amplify the infrared light signal of the remote control being tested. D1 uses a flat infrared photosensitive diode, D2 uses a red round light-emitting diode, R1 is the current limiting resistor of D2, R2 is the current limiting resistor of the IC output on the remote control, and R3 and D1 form a voltage divider circuit to discharge the reverse penetration current of D1. If there is no R3, D2 will be always on. The resistor can be 1/8W.