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. I have not tested it. 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 Level Indicator
T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators. 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.
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
Fluid or Water Level with Reed Relays
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.
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.
This article will explain the way a simple transistor based current source is designed, this will give an idea on how some components can be used in a practical way to make the circuit do some function, the objective is not design but to become familiar with the basic ideas.
Design of a Constant Current Source
In the circuit the LED is used as a reference so to keep it cool a 2.2K is chosen. (20V – 1.6V) / 2.2K = 8.3mA on the high side and when voltage is 10V the current will be 3.8mA min.
You should know that the LED forward drop can change with ambient light as it is photo sensitive and will vary with temperature.
The circuit can be improved by using a zener in place of the LED or better still a temperature compensated reference like LM336.
Operating Current of LM336 is 400uA to 10mA, 20V The max. voltage 20V / 3.3K = 6mA. so within limits. Then you can compute the rest, wire it up to see if your design works.
This Circuit helps in the monitoring of mains supply voltage. It does not use a isolation step down transformer. This has to be constructed only by skilled people with knowledge of safety requirements.
C1 limits the current and drops most of the voltage. The zener regulated supply is for the chip. C2 can be raised to 220uF or more if required. The bar mode display may consume more power.
Mains Voltage Power Transformers
R2-R3-R5-R6 form a voltage divider to get a sample of the input voltage, D11-C3 get the DC value.
Adjust R5 preset with a log Plastic tweaker to get the 5th led to just turn on when input voltage is at 138V AC. This has to be done after PCB is put in a sealed fire-retardant-plastic or epoxy box. drill a hole in box for plastic tweaker.
National LM3914 – Dot Bar Display Driver
This Circuit is Not a Tested Design. It is an Idea for study
Warning : This Circuit is Mains Operated without Isolation Transformer and will give lethal electric shock if touched when the circuit is turned on. Test circuit only with DC 9V Bench Power Supply to try it out. Do not use 230V AC.
This indicates like LM3914 in dot-mode. It is a drawing i made made to troubleshoot a gadget, around two decades ago. Strangely it had a echo of a design i had made into a 7107 dpm years before that. Now i am scanning all my drawing and notes, useful or not. Clean or with errors. Many Errors = 1 Blunder. Some projects i made have been expensive Blunders. So see them with a skeptic eye, fix them, try them. Thats all for now.
See the Circuit Full Size – Microohm Meter with LED Analog Bar
This has a 9V battery power. The 555 spins and a negative voltage for Opamp is created. This is a Low Offset amp of OP37 of Precision Monolithics, Inc PMI an early innovator. This diff-amp amplifies the uV of a 4 wire resistance measurement.
Now the current pump is the 2N2222 you see above the OP37. The FET and 555 do synchronous rectification. The LM324 is the Indicator and Analog to LED Dot-Bar Converter. The probes are Gold Plated, or use solid gold pins if you have them in plenty.
This is a mains 230V AC voltage indicator and is a LIVE CIRCUIT, so take care. The Resistor has to be a fusible ceramic wire wound and the capacitor 630V AC or higher capacity.
More at my Home Made Circuits.
This circuit has been drawn from my memory and i have not tried it out again, just see if it is ok and then try. You should use the fuse of 100mA a slow blow if you want but it is very important. This circuit has to be enclosed in a plastic sealed enclosure to avoid contact.
This is a mains 230V AC load current indicator and is a LIVE CIRCUIT, so take care. The Resistors have to be a fusible ceramic wire wound.
More at Mains Voltage and Power Circuits
This circuit has been drawn from my memory and i have not tried it out again, just see if it is ok and then try. You should use the fuse of 1A a slow blow if you want but it is very important. You can design the shunt R3 and Fuse rating as required by your load.
Note that this circuit is to be put in series with the load like an ammeter. If you put it across the supply like a voltmeter it will fuse out or burn out. This circuit has to be enclosed in a plastic sealed enclosure to avoid contact.
The 555 Astable generates a clock for this circuit, an oscillator giving a square wave output at pin 3 which is counted by 4017 to give a running lights effect.
Digital Timers Counters and Clocks
The decade counter-divider CD4017 has 10 outputs, for every low to high transition at the clock input, rising edge, the counter advances one LED. After going one full circle the the first LED lights again and it goes on. You can vary the value of R2 100K Linear potentiometer to make LEDs run fast or slow.
The frequency of oscillation of astable 555 is given as f = 1.44 / ((R4 + 2 * (R2 + R3)) * C3)
The 10 outputs have 10 green LEDs. The current thru the LED is limited by R1, the current can be calculated like this (9V – 1.6V) / 1K = 7.4mA this is within 20mA which is the danger limit of the CMOS output. You want it to be bright use transistors for every output.
The cap C1 is a filter and C2 is to prevent noise at pin 5 influencing the output as it is a control voltage point.
You can cascade or chain many more counters with the CO or carry out pin 12 of 4017. The pin 15 reset is kept at low for counting, on high it will reset the counter but is not used in this circuit.
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