LOW COST SIMPLE AM RECEIVER CIRCUIT

1. A simple AM receiver circuit based on the MK484 AM receiver IC from Rapid Electronics Ltd is shown here. MK4844 is a monolithic integrated circuit that has all the necessary sections of a AM receiver like RF amplifier, detector, AGC etc. The IC is available in TO92 package and requires very few external components.
2. In the circuit L1 and C1 forms a tank circuit. L1 also serves as the antenna coil. The audio signals available at the pin 3 of IC1 are amplified using the amplifier built around transistor Q1.
3. C2 is a bypass capacitor while C3 is a DC decoupler. Capacitor C4 provides negative feedback while R4 biases the transistor Q1.

NOTES:

• Assemble the circuit on a vero board.
• Use a 1.5V cell for powering the circuit.
• Never give more than 2V to the circuit. It will destroy the IC.
• The MK484 can be operated over a range of 150 KHz to 3 MHz.
• For L1 make 90 turns of 36SWG enamelled copper wire on a 1cm diameter card board former. Experiment with thenumber of turns for getting optimum performance. A little trial  and error is needed.

DEW SENSOR CIRCUIT

1. Here is a simple circuit that can be used to switch ON or OFF a device when the dew present in the surrounding atmosphere crosses a set value.The circuit uses a dew sensitive resistive element and a comparator based on LM 358 to perform the above said operation.
2. At normal condition the resistance of dew sensor element will be low and so the voltage drop across it.So the voltage at the non inverting pin of LM358 (IC1) will be less than the voltage at the inverting input of the LM358
3. So the output of the opamp will be low.This keeps the opto-coupler (MCT2E) deactivated.When the dew increases the resistance of the element increases and so do the voltage across it.
4.  Now the voltage at the non inverting pin of LM358 (IC1) will be higher than the voltage at the inverting input of the LM358.So the output of the op amp will be switched to high.
5. This in turn activates the optocoupler.The LED  glows to indicate it. As a result we get an optopcoupler activated and de activated according to the amount of dew in the atmosphere.The output pins of optocoupler pin (5&4) can be used to control the external device.

NOTES:

• The dew sensor is hard to find in market.But it can be easily obtained from a old VCR.Also the type no of the sensor is not so important here.Try with any thing you get.I used one from a old Hitachi VCR.
• LM 358 is a dual opamp.Here only one opamp inside it is used.

ONE TRANSISTOR CODE LOCK

1. This is of course the simplest electronic code lock circuit one can make. The circuit uses one transistor, a relay and few passive components. The simplicity does not have any influence on the performance and this circuit works really fine.
2. The circuit is nothing but a simple transistor switch with a relay at its collector as load. Five switches (S0 to S4) arranged in series with the current limiting resistor R2 is connected across the base of the transistor and positive supply rail.
3. Another five switches (S5 to S9) arranged in parallel is connected across the base of the transistor and ground. The transistor Q1 will be ON and relay will be activated only if all the switches S0 to S4 are ON and S5 to S9 are OFF.
4. Arrange these switches in a shuffled manner on the panel and that it. The relay will be ON only if the switches S0 to S9 are either OFF or ON in the correct combination.  
5. The device to be controlled using the lock circuit can be connected through the relay terminals. Transformer T1, bridge D1, capacitor C1 forms the power supply section of the circuit. Diode D2 is a freewheeling diode. Resistor R1 ensures that the transistor Q1 is OFF when there is no connection between its base and positive supply rail.

NOTES:

• This circuit can be assembled on a Vero board.
• Switch S1 is the lock’s power switch.
• The no of switches can be increased to make it hard to guess the combination.
• Transistor 2N2222 is not very critical here. Any low or medium power NPN transistor will do the job.

MAGNETIC PROXIMITY SWITCH

1. Here is the circuit diagram of a magnetic proximity switch that finds a lot of applications in many fields.
2. The circuit is based on a magnetic reed switch(S1) as the proximity sensor.
3.  A monostable multivibrator based on NE555 (IC1) and a toggle flip flop based on CD4013 (IC2) does the rest of the circuit.
4.  When a magnet is reached in proximity of S1 it closes to give a negative trigger at pin 2 of IC1.The output of IC1 goes high for a time determines by R2 and C2.
5.  This clocks the IC2 wired as a toggle flip flop.The output (pin 1 ) of IC2 goes high and the transistor Q1 is biased to ON.
6.  This clocks the IC2 wired as a toggle flip flop.The output (pin 1 ) of IC2 goes high and the transistor Q1 is biased to ON.

NOTES:

• Switch S1 can be a general purpose magnetic reed switch.
• The equipment to control can be connected using NC,NO and C points of the relay according to the application.
• Use a 12 regulated power supply for powering the circuit.

BATTERY ELIMINATOR CIRCUIT

1. Here is the circuit diagram of a battery eliminator circuit that can be used as a replacement for 9V PP3 batteries.
2. The circuit given here can be used to power any device that operates from a 9V battery. The transformer T1 steps down the mains voltage and bridge D1 performs the job of rectification.
3.  Capacitor C1 is a filter. IC LM317T is the regulator here.
4.  The value of R1, R2 and R3 are so selected that the output voltage of IC1 will be steady 9 volts.

NOTES:

• Assemble the circuit on a good quality PCB.
• Transformer T1 can be a 230V primary, 9V secondary, 1.5A step down transformer.
• If 1A Bridge is not available, then make one using four 1N 4007 diodes.
• Do not connect loads that consume more than 1.5A to this circuit.
• A heat sink is recommended for IC1.