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LED Voltage Level Indicator

The applications of this circuit are FM tuning indicator, Stereo Balance Indicator, Wire T2 like T1 then we get two channel inputs and battery level indicator.

Muntzing a Circuit Design

This circuit is derived from a Siemens Application Note 1974. This circuit uses common components of today. The circuit is here as it is of high educational value.

You can 'simulate and test' or 'wire it up and try' and let me know how it worked. The Circuit is also a simple analog to digital converter. You can use optos in place of LEDs. 

Battery Monitor for your Boat or Car

T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators. Now to learn more on how they work you have to study circuits at 4QD-TEC.
When input voltage is increased T1 is turned on which leads to more base current for T3 which Lights LED1. When input voltage is less T2 turns on as it gets a better base current from P3 which turns on LED2 via T4. When both LEDs are off T5 gets biased as no drop across R5 which lights the LED3 thru T5 hopefully.

LED Voltage
                        Level Indicator

What you need to know is a small current Ib thru the base-emitter path in the direction of the emitter arrow will lead to a large Current Ic thru the emitter-collector path in direction of arrow. Ic = B * Ib where B - beta is the DC current gain, it could be 100-400.

FET Current Source and MOSFET

Beta is different in each transistor you buy and varies with the test conditions and even with temperature and age. The LED1 and LED2 will indicate above or below Limits set by P2 and P1. The Limit Threshold itself is set at P3 i think. LED3 will light when Hi LED and Lo LED both are off.

Electronic Toss with LEDs

Running LED, Random Results, Decade Counter and Clock. Weigh the Pros-n-Cons, Take a Balanced Decision.

This can be built with one 555 timer and a CD4017. Use the pin 5 to connect to a cap you charge with a pushbutton or gate like CD4093 to turn off 555 using pin 4 after a cap discharges. This may simulate a random.


Press the Red Button above, wait, the LED will spin fast and slow down,then come to rest at the random decision LED. The above Toy may need a VCO and a Ramp Generator as well. Start with this design 555 and CD4017 Running Lights use the 4th LED output to reset the counter.

Colorful LEDS