Telephone status indicator with LED

This will monitor telephone status without loading the telephone line, this way if you have two phones in parallel you will know if one of them is busy. Connect the two ends of circuit in parallel to phone lines.

Telephone LED status indicator

D1 to D4 make a bridge so that LED’s are powered in correct polarity. LED1 indicates line ok no broken line, LED3 can light only when a 12V Zener breaksdown, this shows if line is busy or free, so that you dont go online when someone is talking on the phone in another room. LED2 lights on an incoming call ringing signal.

Learn on Gadget Hacks and Making Things

This is a higher voltage at least in older exchanges, so the cap protects the LED, and LED Lights when a AC ringing signal occurs. D6 protects LED2 from reverse polarity.

Learning Science to Promote Scientific Thinking

Science is best learnt with practical lab work in tandem with theory. Theory becomes appetizing only with practical experiments. In fact theory should follow practical experiments.

Battery Level LED Indicator

This LED Indicator uses a LM339, a quad comparator. LM339 can work on single or dual supplies, it has a open collector output that can drive 15mA, low power consumption. The circuit is an untested design but it should work.

Mains Voltage and Power Circuits – Similar circuits for Mains Voltage Monitoring.

There are many better circuits in the various circuit archives i have linked on the front page, you just have to look around. When you measure the open circuit voltage of a battery with a high impedance DMM (10M), the value may be a bit misleading. Apply a dummy load to bleed the battery a bit so that proper readings can be taken on Load. The load below is a 100 ohms wire-wound fusible ceramic resistor which will heat a bit when you test 12V batteries.
Battery Level LED Indicator

Theory of Operation.

R16 a 5W ceramic wire wound bleeder or dummy load. R15 is a part of an attenuator for obtaining ranges. D2 is a protection clamp diode. R10-D1 forms the 5V reference for comparators. Then an attenuator obtains 1.2, 1.4, 1.6, 1.8 V steps for each comparator. This circuit is similar to Audio Level meter or VU meter circuit.

Comparators in Interface Circuits

The comparator compares the battery sample voltage to the fixed reference step. If ‘+’ pin is more positive than ‘-‘, or is ‘+’ is more dominant, then output goes floating ‘open collector’, so No LED light . But if ‘-‘ is more dominant the output transistor of comparator goes low impedance or saturates or turns ‘ON’. But only spec current can be switched, do not compare with electrical switch ‘ON’. Also on a dual supply 0V is more dominant or positive compared with -12V, even though it appears -12V is a big number. The direction of current is what decides, all measurements are relative.

Simple Mains Voltage monitor

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 0.47uF can be brought down to 0.22uF for low LED currents, use high efficiency ultra bright LEDs.C1 should be 440V AC or 630V DC plastic axial yellow, polyester, polycarbonate, polypropylene, metalized film.

R3-R6-R9-R14-R18 resistor divider determines the LED turn on or threshold switch points, 10M for hysteresis.

Mains Voltage and Power Circuits

Adjust R16 preset with a log Plastic tweaker to get the led D2 to turn on when input voltage is at 220V 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.

LM324 – Low Power Quad Operational Amplifier

This Circuit is Not a Tested Design. It is an Idea for study
Simple Mains Voltage monitor LM324
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.