When you build this circuit you will see what we mean. The effect from the speaker is very similar to a loud ticking clock. The 100mfd electrolytic is very important. It reduces the internal impedance of the battery (especially if it is low) and changes the tone from a soft tick to a very loud sharp click. It also reduces the current from 25ma to about 2ma. The circuit operation is very simple. On connecting the battery the 10k resistor, 200k pot, 2.2 mfd capacitor and 8 ohm speaker are the only parts drawing current. As the capacitor begins to charge the base voltage on the NPN transistor rises to about .6v This turns the transistor on and in turn switches the second transistor to a conducting state. This results in a click from the speaker. At the same time the negative lead of the electrolytic is brought nearer the positive rail and thus the charge on the electrolytic is reduced. This turns off the first and second transistors to begin the cycle over again.
The rate of charge of the electrolytic is dependant upon the value of the resistors in series with it. Thus the rate of ticking can be altered by the trim pot. Almost any NPN and PNP transistors can be used in this circuit. In fact this circuit is an ideal simple test for transistors It will determine if they are NPN or PNP. All the parts are mounted on a small piece of veroboard 15 holes by 15 holes.
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This circuits produces a very handy signal injector. It is a free-running multivibrator with an output of square wave form at a fundamental frequency of about 2kHz and thus is rich in harmonics and can provide a continuous note when injected into any receiver, up to about 20MHz.
The signal injector can be fitted into a plastic tube about 10 to 15 cm long so before cutting the veroboard it is best to find a container.
A small plastic pill bottle is ideal and the veroboard can be cut to size. The probe can merely be a long thin bolt mounted in the centre of the lid.
To find a fault in an amplifier or superhet radio, simply connect the earth lead to the chassis of the amp and move through each stage starting at the speaker. Obviously an increase in volume should be heard at each preceeding stage. This injector will also go through the IF stages of radios and FM sound sections in TV's. Use 2 mercury button batteries for the supply and fabricate a switch into the screw-on lid. The first use for this injector will be to test the operation of the next project...The MINI AMPLIFIER.
This article was taken from Talking Electronics
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