Power Pulser

The idea behind this multipurpose power pulser is very simple. As shown in the circuit (Fig. 1), it uses a low-frequency oscillator to drive a voltage regulator. Timer chip LM555 (IC1) is wired as an astable multivibrator. Components R1 and R2, VR1 and C1 produce the free-running frequency. You can adjust it to some extent by varying potentiometer VR1. The output of IC1 at pin 3 controls the switching on/off of adjustable voltage regulator LM317T (IC2) through npn transistor SL100B (T1).

Fig. 1: Power pulser circuit

You can use input power supply of 5V-18V, 1.5A and adjust the output to 1.25V-15V, 1.5A. This pulsed output can be used for incandescent lamps, DC motors, electromagnetic relays and LEDs.

After selecting the desired load, power up the unit with switch S1 in ‘on’ condition. Now connect a digital multimeter across the output terminal (pin 2) of IC2 and set the required output voltage using potmeter VR2. The frequency of IC1 can be set through VR1, provided switch S1 is ‘on.’ Note that with 18V DC input, the maximum output voltage is approximately 15V only. The frequency of the astable multivibrator can be selected by using values of components R1, VR1, R2 and C1 according to your requirement.

Fig. 2: Pin configuration of regulator lm317

Fig. 3: Proposed cabinet

Assemble the circuit on any general-purpose PCB and enclose in a cabinet as shown in Fig. 3. Connect switch S1 and LED1 on the side of the cabinet. Fix potmeters VR1 and VR2 at the bottom of the front side. Also fix the input and output terminals on the front side of the cabinet. Using external wires, connect the power supply to the input terminal and the load to the output terminal.

Sunset Lamp

LDR-based automatic lights flicker due to the change in light intensity at dawn and dusk. So compact fluorescent lamps (CFLs) are unsuitable in such circuits as flickering may damage the electronic circuits within these lamps. The circuit described here can solve the problem and switch on the lamp instantly when the light intensity decreases below a preset level.

The circuit uses popular timer IC NE555 (IC1) as a Schmitt trigger to give the bistable action. The set and reset functions of the comparators within the NE555 are used to give the instantaneous action. The upper threshold comparator of IC1 trips at 2/3Vcc, while the lower trigger comparator trips at 1/3Vcc. The inputs of both the threshold comparator and the trigger comparator of NE555 (pins 6 and 2) are tied together and connected to the voltage divider formed by LDR1 and VR1. The voltage across LDR1 depends on the light intensity.

In daylight, LDR1 has low resistance and the input voltage to the threshold comparator goes above 2/3Vcc and its output becomes zero, which resets the internal flip-flop of IC1. But the input to the trigger comparator is still more than 1/3Vcc, which keeps output pin 3 of IC1 low. Triac BT136 connected to output pin 3 of IC1 remains quiescent due to insufficient value of current for firing it. Thus lamp L1 remains ‘off’ during daytime.
At sunset, the resistance of LDR1 increases, and the voltage at the input of the threshold comparator decreases below 2/3Vcc and that of the trigger comparator goes below 1/3Vcc. As a result, the outputs of threshold and trigger comparators go high, which sets the flip-flop. This changes output pin 3 of IC1 from low to high. Triac1 gets the necessary gate current through resistor R2 and fires. Thus it completes the power supply to the lamp through Triac1. LED1 glows to indicate the high output state of IC1.

Power supply to the circuit is directly derived from the mains through capacitor C4. This capacitor delivers current in the circuit. Diodes D1 and D2 rectify the AC from capacitor C4 and capacitor C3 provides the necessary smoothing. Zener diode ZD1 provides rectified 15V DC for the circuit. Bleeder resistor R4 removes the stored voltage of the capacitor when the circuit is unplugged.

Assemble the circuit on any general-purpose PCB and enclose in a plug-in type adaptor box. Connect the live and neutral points to the pins of the adaptor box. Provide in the box 5mm holes for LDR1 and LED1. Plug the unit at a place where daylight is sufficient to inhibit the circuit operation during daytime. Light from the lamp should not fall on LDR1 at night.

Caution. The circuit carries 230V AC and most of its points are at mains lethal potential. So do not touch any point in the circuit when it is powered and adjust the preset only with a plastic or insulated screwdriver.

Crystal AM Transmitter

Here is the circuit of a medium-power AM transmitter that delivers 100-150 mW of radio frequency (RF) power.

At the heart of the circuit is a crystal oscillator. A 10MHz crystal is used to generate highly stable carrier frequency. Audio signal from the condenser mic is amplified by the amplifier built around transistors T1, T2 and T3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor T4. Here modulation is done via the power supply line. The amplitude-modulated (AM) signal is obtained at the collector of oscillator transistor T4.

Fig. 1: Circuit of crystal AM transmitter

Fig. 2: Oscillator coil

Fig. 3: Modulation transformer

By using matching dipole antenna and co-axial cable, the range of signal transmission can be increased. For maximum range, use a sensitive radio with external wire antenna.

The circuit works off a 9V-12V battery. For oscillator coil L1, wind 14 turns of 30SWG wire round an 8mm diameter radio oscillator coil former with a ferrite bead (see Fig. 2). For modulation transformer X1, you can use the audio output transformer of your old transistor radio set. Alternatively, you can make it from E/I section transformer lamination with inner winding having 40 turns of 26SWG wire and the outer winding having 200 turns of 30SWG as shown in Fig 3.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet.

Hum-Sensitive Touch Alarm

Radiation signals from mains wiring can travel a few metres of distance. These can be induced by the electromagnetic field in the human body also.

This touch-sensitive alarm is based on generation of the AC hum signal. When someone touches the touch plate, low-power AC hum (the same as induced from AC wiring of the house) is generated on the touch plate. This signal is first amplified by the high-gain preamplifier built around IC 741 (IC1) and then fed to another op-amp CA3140 (IC2) that is wired as a voltage comparator.

When IC2 receives the amplified signal at its input pin 3, its output goes high. As a result, transistor T1 conducts to sound the buzzer. At the same time, LED1 glows. For satisfactory working of this unit, power it from mains- derived 12V.
Assemble the circuit on a general-purpose PCB and enclose in a small cabinet. Use a shielded wire to connect the touch plate to the circuit keeping the length of the wire and the size of the touch plate as short as possible.

Hot-Water-Ready Alarm

Electric kettles turn off automatically when water has boiled. What if the boiler beeps to alert you when your water has boiled? The tripping sound of the thermal switch may not register as an alarm in your mind. Here is such an add-on unit that gives intermittent beeps at the end of boiling. It has the advantages of extremely low component count, low cost, small size and light weight. Fig. 1: Hot-water-ready alarm circuit In this circuit, current flowing through the coil provides a magnetic field that actuates a reed switch. The coil is made of twelve turns of 20SWG enamelled copper wire to carry the current (refer Fig. 3). The number of turns will change with the wattage of the kettle and the type of reed switch. Fig. 2: Reed switch Fig. 3: Reed switch with coil When the kettle is switched on, current through the coil creates a magnetic field, the reed contact closes and C2 charges through R1. Although the reed switch periodically opens at AC mains frequency, the time constant of R1 plus R2 and C1 is such that there is no voltage across C1. When the kettle switches off, the reed switch is permanently open and capacitor C1 starts charging through resistor R2. This capacitor C1 voltage makes transistor T1 conduct and the buzzer sounds (as it gets the supply path) to indicate that water has boiled. Fig. 4 shows a piezobuzzer. Fig. 4: Piezobuzzer Assemble the circuit on a common PCB and enclose in a plastic case. Connect mains input power supply to the circuit and 230V AC output to the kettle. Fix the unit near the kettle. Connect reed switch and its coil as shown in the circuit.