Voltase hobby

This schema lets you turn on/off a fan by just directing torchlight or other light toward its light-dependent resistor (LDR). The schema is powered from a 5V power supply.

Preset VR1 and a light-dependent resistor (LDR) work as the potential divider. Normally, the LDR’s resistance is high (20 kilo-ohms) in darkness and low (2 kilo-ohms) in light. This value of high and low resistances varies for other LDRs. Preset VR1 is used for setting the intensity of light, while preset VR2 is used for setting the output time period of IC1.

Fan on/off Control by Light Circuit Diagram

When light falls on the LDR, the monostable (IC1) triggers at pin 2, making its output at pin 3 from low to high. This low-to-high transition forms a clock for D flip-flop. The D flip-flop is operated in toggle mode by connecting its Q output to D point. The flip-flop output goes to an inverter (N1). The inverter output is fed to the relay driver transistor.

When the inverter output is low, diode D1 conducts and the current is diverted into the inverter. Hence the relay does not energise. When the inverter output is high, diode D2 conducts and the current is diverted into transistor T. Hence the relay energises.

One terminal of the fan is connected to the normally-open (N/O) contact of the relay, while another terminal is connected to the neutral (N) of mains. The mains live (L) is connected to the pole of the relay. When the relay energises, the fan turns on. Otherwise, the fan remains off.

Switches S1 and S3 are for initial resetting of the monostable (IC1) and D flip-flop (IC2), respectively, and switch S2 is used for setting the D flip-flop. Paste a piece of paper on the face of the LDR so that it doesn’t get activated by ambient light. Use a torch to light the LDR.

After initial resetting of the monostable and D flip-flop, the inverter output goes high and the fan turns on via the relay. When light falls on the LDR, the fan goes off. If torchlight is again directed toward the LDR, the fan turns on. The sequence repeats.

Initially if switch S2 is used to set the D flip-flop, the fan is held ‘off’. The relay does not energise as the Q output of D flip-flop goes high to make the inverter output low. Directing the light towards the LDR at this moment turns the fan ‘on.’

Sourced By: EFY Author: V. Gopalakrishnan

The 30 watt amplifier schematic shown below provides an appropriate power boost with an input of 4 watt up to 6 watts. The schema is designed to cover 88-108MHz FM Broadcast Band. However, the schema is very stable at my place and provides a clean-output through seven (7) element Butter-worth low-pass filter.

The heart of the schema is 2SC1946A VHF RF power transistor. The transistor is specifically designed for operation in frequencies up to 175 MHz, with very good results. As you can see, the power line is well decoupled. The amplifier current can be over 5 amps. All the coils are made from 16gauge laminated wire (or Silver copper wire can do best) and the RFC can be of HF toroid core (as shown in the picture) or 6 holes ferrite bead.C3 and R1 forms snubber schema while R2 and C6 prevent the amplifier from self-oscillation at VHF, sometimes you need to add 180 ohms in parallel with L7.That will cause the amplifier to dissipate UNDESIRABLE VHF thereby reducing spurious level.

The photo below is 60Watts VHF power amplifier using the above schema. Two of 2SC1946A transistors are arranged at 90 degrees to each other and their outputs are combined using "Power Combiner Network”. It is quite difficult to combine powers at VHF and UHF bands.

However, I recommend that hobbies should stick to single power design due to its complicity and large rate of INTERFERENCE. (in attempt to go for double transistors which involves power combiner network). Since the two amplifiers are operating in different phase (out of phase).

Tuning of the amplifier is not hard at all. You just have to connect the output to a good antenna with a transmission line (RG214) of 50 ohms. First match the output network, and then do the same to the input network for a maximum power output. By way of adjustment, you can increase the output at its operating frequency.