Showing posts with label wave. Show all posts
Showing posts with label wave. Show all posts
Saturday, November 8, 2014
Build a Radio Wave Alarm
This simple circuit is sure to have the police beating a path to your door- however, it has the added advantage of alerting you to their presence even before their footsteps fall on the doormat.Simple Radio Wave Alarm Circuit Diagram :
Notes :
- The circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b. When a hand or a foot approaches the sensor, the frequency of the transmitter (IC1a) drops appreciably.
- Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is tuned to a frequency just below this. The 1MHz transmission will therefore not be heard by the radio. But bring a hand or a foot near to the sensor, and the transmitters frequency will drop, and a beep will be heard from the radio.
- Attach the antenna to a multiplug adapter that is plugged into the mains, and you will find that the Medium Wave transmission radiates from every wire in your house. Now place a suitably tuned Medium Wave radio near some wires or a plug point in your house, and an early-warning system is set up.
- Instead of using the sheet of tin foil as the sensor, you could use a doorknob, or burglar bars. Or you could use a pushbutton and series resistor (wired in series with the 33K resistor - the pushbutton would short it out) to decrease the frequency of IC1a, so activating the system by means of a pushbutton switch. In this case, the radio would be tuned to a frequency just below that of the transmitter.
Thursday, September 11, 2014
Short Wave Converter
This short-wave converter, which doesn’t have a single coil requiring alignment, is intended to enable simple medium-wave receivers to be used to listen to short-wave signals. The converter transforms the 49-m short-wave band to the medium-wave frequency of 1.6 MHz. At the upper end of the medium-wave band, select an unoccupied frequency that you want to use for listening to the converted short-wave signals. Good reception performance can be obtained using a wire antenna with a length of one to two metres.
Short-Wave Converter Circuit Diagram
The converter contains a free-running oscillator with a frequency of around 4.4 MHz, which is tuned using two LEDs (which act as variable-capacitance diodes!) and a normal potentiometer. The frequency range is set by adjusting the emitter current using a 1k trimpot. The oscillator frequency depends strongly on the operating point. This is due to the combination of using an audio transistor and the extremely low supply voltage. Under these conditions, the transistor capacitances are relatively large and strongly dependent on the operating point. The second transistor forms the mixer stage.
If you calculate the resonant frequencies of the tuned diagram, you will obtain 6.7 MHz for the antenna schema and 1.7 MHz for the output schema. Additional transistor capacitance and the effects of the coupling capacitors shift each of the resonant frequencies downward. The tuned diagram are relatively heavily damped to obtain bandwidths that are large enough to allow the schema to be used without any specific alignment. The results are good despite the low collector–emitter voltage of around only 0.6 V, due to the fact that only a modest amount of mixer gain is necessary. The entire schema also draws less than 1mA.
If you calculate the resonant frequencies of the tuned diagram, you will obtain 6.7 MHz for the antenna schema and 1.7 MHz for the output schema. Additional transistor capacitance and the effects of the coupling capacitors shift each of the resonant frequencies downward. The tuned diagram are relatively heavily damped to obtain bandwidths that are large enough to allow the schema to be used without any specific alignment. The results are good despite the low collector–emitter voltage of around only 0.6 V, due to the fact that only a modest amount of mixer gain is necessary. The entire schema also draws less than 1mA.
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