Battery Tester for 1.5 and 9V

Parts List:
R1 = 18K
R2 = 240 Ohm
R3 = 8.2K
R4 = 3K
R5 = 10 Ohm
M1 = Panel Meter (Anyone will work)

Design Considerations:
You may have experiment with the values of R3 and R4 to get an accurate reading from the meter. Every meter is different, so a little bit of playing with the resistor values is required.Try using a variable resistor in place of R3 & R4 to get a value of resistance that works.

By. Matthew B.

Battery Monitor for 12 Volt Lead-Acid

This simple circuit makes it posible to monitor the charging process to a higher level.Final adjustsments are simple and the only thing needed is a digital voltmeter for the necessary accuracy.Connect an input voltage of 12.65 volt between the positive and negative poles and adjust the 10K trimmer potentiometer until Led 10 lights up. Lower the voltage and in sequence all other Led's will light up. Check that Led 1 lights up at approximately 11.89 volts.At 12.65 volt and higher the battery is fully charged, and at 11.89 is considered 'empty'.The green Led's indicate that the battery capacity is more than 50%, the yellow Led's indicate a capacity of 30% - 50% and the red Led's less that 30%.

This circuit, with the components shown, uses less than 10mA. Ofcourse you can adapt this circuit to your own needs by making small modifications. The circuits above is set for 'DOT' mode, meaning only one Led at a time will be lit. If you wish to use the 'BAR' mode, then connect pin 9 to ground, but obviously with increased current consumption.The LED brightness can be adjusted up- or down by choosing a different value for the 4K7 resistor connected at pin 6/7You can also change the to monitoring voltage level. For example, let's say you wanted to change to 10 - 13 volt, you connect 13volt to the input (+ and -) and adjust the 10K potentiometer until Led 10 lights up. Change temporarily the resistors at pin 4 with a 200 Kilo-ohm potentiometer and reconnect a voltage from 10 Volt to the input. Now, re-adjust the 200K potentiometer until Led 1 lights up. When you are satisfied with the adjustment, feel free to exchange the 200K potentiometer with resistors again.(after measuring the resistance from the pot, obviously).

The diode 1N4007 was included to protect the circuit from a wrong polarity connection.It is however strongly recommended to connect the monitor directly to the battery, in principle a connection to the cigarrette lighter would suffice but for reasons unknown at this time the voltage at that point is 0.2 volt lower than the voltage measured directly on the battery. Could be some residual resistance caused by ignition switch and path through the fuse?

By. Jan Hamer

Basic RF Oscillator

This basic circuit is easy to build and the components are not critical. Most of them can be found in your junk parts box. The L1 antenna coil can be made by close winding 8 to 19 turns of 22 gauge insulated hookup wire around a ¼ inch fro such as a pencil. You can experiment with the size of the coil and the number ofof turns to see how it effects the frequency and signal output of the oscillator. You should be able to pick up its signal with a standard FM radio receiver. The “Signal” should be coupled by disc capasitor of about 0.1µF to the stage in front of it.

By. “antoon

Audio PreAmplifier

This easy circuit provides good gain to weak audio signal. Use it in front of an RF oscillator to make an RF transmitter that very sensitive to sound.


This preamp is very simple, and will work under tough conditions. The input impedance is pretty high, so it won't load down electric guitars. It will also amplify microphones. When I have a moment, I'll make some additions that will optimize it for microphones, and also show how to power a condenser microphone.

Source : Circuits for the Hobbyist