Simple Electronic Combination Lock Circuit

This is a design circuit for a very easy and simple electronic combination lock based on IC LS7220. This could be the circuit diagram of a easy electronic combination lock by using IC LS 7220.This circuit may be applied to activate a relay for controlling (on & off) any device each time a preset combination of 4 digits are pressed. The circuit may be operated from 5V to 12V. This is the figure of the circuit;

To set the mixture connect the appropriate switches to pin 3,4,5 and 6 of the IC through the header. As an example if S1 is connected to pin 3, S2 to pin 4 , S3 to pin 5, S4 to pin 6 of the IC ,the combination is going to be 1234.This way we can create any 4 digit combinations. Then connect the rest of the switches to pin 2 of IC. This will cause the IC to reset if any invalid key is pressed , and entire key code has to be re entered. When the correct key combination is pressed, then the output ( relay) will be activated for a preset time driven by the capacitor C1.Here, it is set to be 6S.Increase the value of C1 to increase the preset time.

For the keypad, arrange the switches in a 3X4 matrix on a PCB (Printed Circuit Board).Write the digits on the keys by using a marker. Instead of applying numbers I wrote some symbols!.The bad guys is going to be more confused with this.

Component Part List:
·    C1 = 1uF 25V
·    C2 = 220uF 25V
·    R1 = 2.2K Ohm
·    Q1 = 2N3904 / 2N2222
·    D1 = 1N4148 / 1N4001-1N4007
·    K1 = 12V SPDT Relay / Any appropriate relay with 12V coil
·    U1 = LS7220 Digital Lock IC
·    S1-S12 = SPST Momentary Pushbutton  Keypad (see notes)
·    HD1 = 12 Position Header

Simple Adjustable Sine/Square Wave Oscillator Circuit

This is a design circuit for simple easily tuned / adjustable sine and square wave oscillator. This is the figure of the circuit;

This circuit provides sine and square wave at frequency of below 20Hz up to above 20KHz. The benefit of this circuit diagram is that you can adjust the output frequency by varry the variable resistor of R6.

Simple Ni-Cd Battery Discharge Limiter Circuit

This is a very simple Ni-cd battery discharge limiter electronic project can be designed like in this circuit diagram using few common electronic parts. This circuit is very simple and can be used in many compact designs. This circuit disconnects the battery from the load when the output voltage falls bellow a preset level. C1 charges through R1 and turns on Q2. This is the figure of the circuit;

Collector current flows through R2 turning Q1 on and battery is connected to the load. When the output voltage falls bellow a point set by RV1, Q2 turns off, Q1 turns off and further discharge of the battery is prevented. The maximum output that can be supported by this discharger circuit is around 3 amperes. At the input you can connect on or two 12 volt Ni-Cd batteries.

Simple Ham Radio Circuit For Audio Filter

One of the earliest modes of radio communications is the use of Morse Code on a continuous wave carrier (CW) which is interrupted to generate a signal in forming an alphabet. Though it looks simple, this method is often makes less reliable as the surrounding man-made noise and atmospheric noise caused interference to the receiver station. This project will help to filter out the interference signal and ensure that the signal received from the Morse code station stand out. The circuit design of the filter circuit is simple and easy to build. Figure below shows the filter's selectivity response curve. It is sharp at the peak at 20dB at the audio frequency of 800kHz - 900kHz;

It uses common parts that can be purchased easily from any electronic shops. At the core of the circuit is a OP AMP where its input is connected to the headphone jack of the receiver. Some of the amplified signal is feedback through potentiometer VR1 to the non inverting input which helps to boosts the gain and hence called positive feedback. Some is passed to the inverting input through a LC circuit. This cancels the gain of the amplifier except at the resonant frequency - high impedance at resonance state. The amount of positive feedback is adjusted using the potentiometer VR1. 

L choke should be in the region of 5H but this is not critical. Try using the winding of an audio transformer to make this choke. The output of this filter is connected to the earphones. On Switch SW and on the radio. Vary the VR1 until it goes into oscillation and then turn it back a bit. Tune across the code station and you will hear a sharp peak response effectively cutting off other noise and interference.