Snap Circuits K-24 - Manual (Page 4)

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MINIATURE RADIO TRANSMITTER
The Whooper Alarm puts out a wavering sound that is sure to startle an
intruder. It can be used independently or as an accessory to the Burglar
Alarm Kit K-23.
The Whooper Alarm circuit consists of two oscillators, a low frequency
oscillator which drives a higher frequency unit at a predetermined rate.
The high frequency oscillator drives an output transistor which powers the
speaker.
CIRCUIT OPERATION
Figure 1 shows the circuits of the low frequency oscillator. When the
power is first applied to this circuit, transistors Q1 and Q2 will not conduct.
This is because the base of transistor Q2 is about 5.4V while the emitter
is at zero volts.
A current is flowing in resistor R2 charging capacitor C1. When the voltage
across C1 reaches 6V, transistor Q2 starts sending a current in the
collector of Q1. The current in the collector of Q1 is mulitplied by the gain
of transistor Q1 and this rapidly turns on transistor Q2. Capacitor C1
quickly discharges through resistor R8 as shown in Figure 2. Note that C1
charges through a 2200
W
resistor R2, but discharges through a 100
W
resistor R8. Thus, the charge to discharge ratio is 22:1. When C1 is
discharged, Q1 and Q2 turn off and the whole cycle repeats itself.
HIGH FREQUENCY OSCILLATOR
The circuit of the high frequency oscillator is shown in Figure 3.
Transistors Q3 and Q4 are wired as amplifier stages. The bias for these
amplifiers are controlled by the sawtooth of Figure 2. These amplifiers
normally would amplify the low frequency pulses, except for the addition of
capacitor C3. This capacitor takes the output of Q4 and feeds it in phase
to the input of Q3. This causes the circuit to oscillate. The frequency of
oscillation is controlled by the RC time constants of C3 and R6. The
frequency of oscillation is about 1,000 cycles per second. This frequency
is modulated with the low frequency oscillations to produce the Whooper
Alarm sounds. Transistor Q5 further amplifies the signals and drives the
speaker.
Figure 1
Figure 2
Figure 3
Volts
Time