Simple Low Voltage Alarm Circuit

This is a circuit for alarm circuit. This is a simple form circuit. This is the figure of the circuit;

This circuit can be used to monitor batteries and other volatile sources of current for problems. The circuit sounds an alarm and lights an LED, but can be interfaced to any number of other circuits for many different uses.
Part;
R1, R3 1K 1/4W Resistor
R2 5K Pot
U1 LM339 Voltage comparator IC
D1 1N5233B Zener Diode
D2 LED
BZ1 Piezo Buzzer
MISC Board, wire, socket for IC

Simple Servo Tester Circuit

This is a circuit for a simple servo tester which will comprehensively test the capabilities of almost any modern servo. It has two pushbuttons, CENTRE and SWEEP and a potentiometer. This is the figure of the circuit;

CENTRE Does exactly that, centers the servo, afterwards the potentiometer determines position. SWEEP Sweeps the servo back and forth at a rate determined by the potentiometer setting. The PIC uses its internal timer to set up a constant frame duration of 20ms and the on/off ratio is set by the user.

Simple Optical Switch Circuit

This is a circuit for optical switch that is used 555 timer IC. The IC is proved to be the most versatile and ubiquitous IC all over the world. This is a possible use: simple inverting schmitt trigger. This is the figure of the circuit;

When the phototransistor is stroke by IR light it conducts and the voltage between the 1Mohm resistor(arbitrary) and the photo trans drops from VCC to lower values. When the voltage drops lower than VCC/3 the 555 is triggered and goes high (from 0 TO VCC). The amount of light that strike the photo trans necessary to bring his collector to VCC/3 is determined by the resistor (Vdrop = Icollector * R , so , if Vdrop= 2*VCC/3, the resistance needed to set the threshold on current is R=2*VCC/(Icollector*3)). High sensibility photo trans would need a smaller resistor, and weaker phototransistors higher value resistor, you can also use a trimmer to set the on threshold level with precision. The time of phototransistor isn't critical. The 555 has high current capability and can drive various devices, such as Bipolars, relays, bipolar + relays, mosfets, mosfets + totem pole , or give a logic output.

In case you need to trigger something when the gate is blocked (for example a burglar alarm, or a multistage coil gun) you need to invert the output, which is accomplished using a small bipolar transistor wired in an inverting setup (see pic) or swapping the positions of phototransistor with the resistor, so the voltage will drop under VCC/3 when blocked: The formula to determine the resistance to turn off at Icollector is R=VCC/(Icollector*3).

Simple Bass Booster Circuit

This is a circuit for bass booster that is powered by an ordinary 9 volt transistor radio battery. This is the figure of the circuit;

The design of a Bass Booster Circuit using LM390. The LM390 Power Audio Amplifier is optimized for 6V, 7.5V, and 9V operation into low impedance loads. The booster connects between your instrument and its amplifier through two standard RCA Jacks.

Auto Heat Limiter Circuit for Soldering Iron

Usually a soldering iron takes a couple of minutes to get adequately heated up to melt the solder, after which the heat generated is much above the requirement and is wasted. This is a design circuit for the solution of the problem. This is a heat limiter circuit that can be work automatically. This is the figure of the circuit;

Once the main is switched on, an approximate 15v drop of the positive half cycle across R5 is detected and supplied to Q1 (SL100 or D313), which acts as a voltage regulator. Zener diode D2 together with diode D3 (yellow LED) stabilizes the emitter voltage of Q1 at 13.2Vdc, which is then delivered to the relay circuit built around Q2 and C3. Capacitor C3 charges through the base-emitter path of Q2 and causes the relay to actuate, which in turn allows both the half cycles of the AC mains to flow through diode D6 and R5 to the load to heat it up at a normal rate.

After a certain lapse of time (about 2 minutes preset) C3 saturates and Q2 stops conducting through the relay, thus switching on series diode D5 to allow only half of the Ac cycle through the load. After switching off the system, C3 discharges very slowly through R2 and R3. Before C3 gets completely discharged, if the power is switched on again, C3 takes a shorter time to reach the saturation level, thus switching series diode D5 much earlier than the preset time to prevent double heating of the load.

However, if the circuit is switched on only after a few seconds of switching off, C3 gets no time to discharge and the relay does not actuate at all. Moreover, if the relay circuit fails due to any reason and Q2 does not conduct, no harm is done to the load because in that case D5 remains in series with it. Thus the circuit offers complete protection to the load.

As stated earlier, the given value of C3 gives a delay of 2 minutes. However, a 1000mfd capacitor can also be used to produce a 4.5-minute delay. R5 maintains a drop of about 15V across itself.

Simple Mini Variable Power Supply Circuit

This is a circuit for variable power supply. This circuit is a simple form circuit. This is the figure of the circuit;

T1 steps down AC voltage from 115VAC (or 220VAC) to about 8VAC and is then rectified via bridge rectifier BR1 to about 11.52Vdc. C1 filters off the AC ripple. If you find the circuit output too noisy add another electrolytic capacitor over the output terminals. Value can be between 10 and 100uF/25V. The output voltage is variable with the 10K-potentiometer while keeping the current constant.

Simple Electronic Drum Synthesizer Circuit

This is a circuit for electronic drum synthesizer. This circuit is consists of two “twin-T” oscillators set to a point below oscillation. To set the circuit into oscillation, we can touch the Touch Pad. This is the figure of the circuit;

By touching the pads in different ways and a whole range of effects are available will produce different effects. Before oscillation, the two 25k pots are adjusted to a point. We can produce a drum roll by shifting a finger rapidly across adjacent ground and drum pads.

Simple Discrete Logic Probe

This is a design circuit diagram for logic probe circuit. Logic probe is used to determine if a point in a circuit has high or low state when the circuit is in operation. This is the figure of the circuit;

When the probe is not touching any circuitry, this circuit consumes no current. It can happen because the voltage across the red LED and base-emitter junction of the 2N3904 plus voltage across the green LED, the base-emitter junction of the 2N3906 is approx 1.7v + 0.6v+2.1v + 0.6v = 5v. This voltage is greater than the supply voltage. the red LED will  illuminate, when the circuit detects a HIGH (above 2.3v) and When detects a LOW is detected, 2N3906 will turn on and the green LED illuminates.

Simple 1N4007 Silicon Diode Laser Power Supply Circuit

This is a circuit power supply. This circuit is design for laser. This is the figure of the circuit;

If you have ever worked with lasers, you know how fun and interesting it can be, you also know how expensive it can be. The high voltage power supplies for the laser tubes are often more expensive then the tubes themselves. This supply can be built with commmon parts, most of which you probably already have in your junk box. The secret is the transformer used. It is a common 9V 1A unit, connected backwards for step up.