Saturday, January 11, 2014

Cheap Bicycle Alarm Schematics

The author wanted a very cheap and simple alarm for some of his possessions, such as his electrically assisted bicycle. This alarm is based on a cheap window alarm, which has a time-switch added to it with a 1-minute time-out. The output  pulse of the 555 replaces the reed switch in the window alarm. The 555 is triggered by a sensor mounted near the front  wheel, in combination with a magnet that is mounted on the spokes. This sensor and the magnet were taken from a cheap bicycle computer.

Circuit diagram :
Cheap Bicycle Alarm-Circuit Diagram
Cheap Bicycle Alarm Circuit Diagram

The front wheel of the bicycle is kept unlocked, so that the reed  switch closes momentarily when the wheel turns. This  triggers the 555, which in turn activates the window alarm. The circuit around the 555 takes very little current and can  be powered by the batteries in the window alarm.  There  is just enough room  left inside the enclosure of the window  alarm to mount the time-switch inside it.

The result is a very cheap, compact device, with only a single cable going to the reed switch on the front wheel. And the noise this thing produces is just unbelievable! After about one minute the noise stops and the alarm goes back into standby mode. The bicycle alarm should be mounted in an inconspicuous place, such as underneath the saddle, inside a (large) front light, in the battery compartment, etc.

Hopefully the alarm scares any potential thief away, or at least it makes other members of the public aware that something isnt quite right.

Caution. The installation and use of this circuit may be subject to legal restrictions in your country, state or area.

Continue Reading[...]

Friday, January 10, 2014

Long Range Direct Conversion Receiver

     Using the circuit of direct-conversion receiver described here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band.

    The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifier-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (variable frequency oscillator) whose output is injected into the gate of first FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency note in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios.

    The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker.

    The receiver can be powered by a 12-volt power-supply, capable of sourcing around 250mA current. Audio output stage can be substituted with a readymade L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used in the circuit is given in the circuit diagram itself.

Continue Reading[...]

30V Variable Power Supply Using LM317

This 30v variable power supply circuit is based on LM317  voltage regulator circuit . This LM317 30v variable power supply circuit can deliver high current (around 5 amps) and variable output voltage between 1.2 volts, up to 30 volts. The led D3 mounted on pin 6 at lm301 lights in constant current mode .

Circuit diagram 

Current limit can be adjusted using R2 potentiometer and the output voltage can be adjusted from 1.2 volts to 30 volts using R8 potentiometer . Input voltage for this variable power supply must be around 35 volts .For this power supply circuit you need to use LM317K circuit (in to3 package ) which must be mounted on a heatsink .
Continue Reading[...]