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CIRCUIT OPERATION
Basically, the Electronic Cricket is a very high frequency oscillator which is turned on for a very short period of
time, which makes it very difficult to find.
Figure 1 shows the block diagram of the Electronic Cricket. The circuit consists of two oscillators, a very low
frequency one cycle every two minutes and a very high 10,000 cycles per second. The low frequency oscillator
controls the high frequency oscillator and will allow it to oscillate for about 5 seconds every two minutes. The
circuit will automatically turn off the 10,000 cycles oscillator until it’s ready for the next cycle.
OPERATION OF THE LOW FREQUENCY OSCILLATOR
Figure 2 shows the schematic diagram of the low frequency oscillator. When the power is turned on, a current
(I1) will flow through resistor R1, turning on the base of transistor Q1. This in turn produces a much larger
current (I2) in the collector of Q1 and into the emitter base of Q2, turning transistor Q2 on very strongly. Now,
the collector of Q2 produces a comparatively larger current I3, rapidly charging capacitor C1. C1 sends a
current into the base of Q1 which further increases current I2 and I3. This action is known as positive feedback
and occurs in about 5 seconds. Note that C1 charges through the 22,000 ohm resistor R2. Once this happens,
current in C1 reverses itself turning transistor Q1 and Q2 off. Capacitor C1 reverses itself turning transistor Q1
and Q2 off. Capacitor C1 now must discharge through R2, R3 and R1. These values add up to 1,023,500 ohms
or a very long time compared to the 22,000 ohm charging resistance. The result is a very short on time and
very long off time for transistor Q1 and Q2 as shown in Figure 3.
OPERATION OF THE HIGH FREQUENCY OSCILLATOR
Refer to the schematic diagram on page 5. Note that transistor Q3 and Q4 form a second oscillator very similar
to oscillator Q1 and Q2. The only difference is the value of capacitor C2 and resistor R5. C2 is much smaller
in capacitance. This causes the oscillator to oscillate at a much higher frequency, 10,000 cycles per second.
Also, note that the input of Q3 is driven from the same point as the input of transistor Q1. (R1 is very high in
value and not important for this consideration).
This results in transistor Q3 being on when Q1 is on,
remembering Q1 is on only for about 5 seconds every two minutes. Therefore, oscillators Q3 and Q4 will only
oscillate for 5 seconds every two minutes (refer to Figure 4). The resulting burst of oscillation are fed to the
speaker, which is then heard as our mysterious noise.
If you wish to change the duration of the burst, you can change the value of resistor R2. A larger value will
lengthen the burst. To change the time between bursts, you must change the value of resistor R1. Lowering
the value of R1 will decrease the time between bursts. Resistors R4 and R5 will change the frequency of the
high frequency oscillator.
Figure 1
Figure 2
Figure 3
5 SEC
2 MIN
ON TIME
OFF TIME
I1
I2
I3
OUTPUT
C1
R2
R3
R3
Q1
Q2
R1
9V
+
+
5 SEC
2 MIN
