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Description:
Ultrasonic Burglar Alarm: Transmitter
and receiver parts of the circuit are connected each other by using a
triple cable. System operates with 20V supply and 10V effective output
voltage drives the ultrasonic transducer at its highest value.
Transmitter circuit gives the maximum gain at 40kHz.
Ultrasonic
transducers in the circuit must be placed vis-à-vis and when a barrier
passes between them it must block the connection without affected from
reflections.
Transmitter is continuously operating and generating
40kHz ultrasonic oscillations. Receiver is designed to operate only when
no ultrasonic signal reaches it and it produces an output signal. You
can connect an alarm system with a relay to the output of the receiver.
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Description:
Ultrasonic
oscillations which are generated by unmodulated ultrasonic transmitters
commonly for remote control are usually affected from other ultrasonic
sources in the media and that's why we are giving a 500Hz modulated
ultrasonic transmitter circuit here.
Voltage supply of the
circuit is 9 Volts and any change of the value will change the
frequency also. You can omit this difference by connecting a resistor
between A and B nodes. You can find the value of the resistor by using
this formula:
R = (Vsupply - 7V) / O.6 Kohm where Vsupply is the new supply voltage value.
The
multivibrator generates 500Hz modulation frequency. To avoid the
frequency differences , tolerance of the circuit should not exceed %5.
If the circuit components changed, then the frequency can be determined by using the formula below,
f = (1.44 x 1000) / (C2 x R3 x C3 x R2)
If C1 and C3 are nanofarads then R2 and R3 will be magaohms.
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Description:
Normally, in a digital voltmeter, high input impedance of the preamplifier stage is protected against very high voltages by two diodes. One of them is connected to the positive terminal and the other one is connected to the negative terminal of the power supply. Practically this kind of protection is very sufficient. But the diodes that are used for this process must have very low leakage current values.
The main problem is that kind of diodes are expensive and it is sometimes hard to provide them. On the other hand electronic hobbyists commonly choose general purpose diodes like 1N4148. But this situation causes to occur some millivolts on 1M input impedance. Today, it is usually very important to measure the values with high accuracy so this extra voltages will affect our experimental results...
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