My Circuit

This simple circuit can indicate a misuse or tapping of Telephone line through a loud alarm. The circuit is too simple and can be easily assembled on a common PCB. Line voltage of Telephone lines is around 48 volts DC in the On hook state. When the handset is lifted, this voltage reduces to 12 volt DC. This change in voltage level is used to activate the circuit.When the switch S1 is closed, circuit becomes active and the telephone enters into the armed state. The high volt DC from the telephone line passes through R1 and VR1 and bias T1 into conduction. As a result, the collector of T1 goes to ground potential to inhibit T2 from conduction. Buzzer and LED thus remain off. When the handset is lifted, the DC voltage from the telephone lines drops to 12 volts. This turns off T1 and T2 conducts. Buzzer beeps and LED lights indicating that the telephone is using. Setting Connect the circuit to Telephone lines using a telephone plug. The free socket of the telephone or Caller ID can be used...

LM3914. The heart of this circuit is the LM3914 from national semiconductors. The LM3914 can sense voltage levels and can drive a display of 10 LEDs in dot mode or bar mode. The bar mode and dot mode can be externally set and more than one ICs can be cascaded together to gat an extended display. The IC can operate from a wide supply voltage (3V to 25V DC). The brightness of the LEDs can be programmed using an external resistor. The LED outputs of LM3914 are TTL and CMOS compatible. Description. In the circuit diagram LEDs D1 toD10 displays the level of the battery in either dot or bargraph mode. Resistor R4 connected between pins 6,7 and ground controls the brightness of the LEDs. Resistors R1 and POT R2 forms a voltage divider network and the POT R2 can be used for calibration. The circuit shown here is designed in order to monitor between 10.5V to 15V DC. The calibration of the circuit can be done as follows. After setting up the circuit connect a 12V DC source to the input. Now adju...

Simple, two-wire, remote monitoring unit, Three-LED level display, 9V battery powered. The whole project was developed on a friend's request. Its purpose was to remotely monitor the water-level in a metal tank located in the attic by means of a very simple control unit placed in the kitchen, some floors below. Mains requirements were: No separate supply for the remote circuit Main and remote units connected by a thin two-wire cable Simple LED display for the main unit Battery operation to avoid problems related to mains supply and water proximity As the circuit was battery operated a low current consumption was obviously welcomed The very small remote unit is placed near the tank and measures the water level in three ranges by means of two steel rods. Each range will cover one third of the tank capacity: Almost empty - signaled by means of a red LED (D3) in the control unit display About half-level - signaled by means of a yellow LED (D2) in the control unit display Almost full -...

Power On Indicator Circuit diagram. Some types of electronic equipment do  not provide any indication that they are  actually on when they are switched on.  This situation can occur when the back-light of a display is switched off. In addition, the otherwise mandatory mains  power  indicator  is  not  required  with  equipment  that  consumes  less  than  10 watts. As a result, you can easily forget  to switch off such equipment. If you want  to know whether equipment is still drawing power from the mains, or if you want  to have an indication that the equipment  is switched on without having to modify the equipment, this circuit provides a solution.  One way to detect AC power current and  generate a reasonably constant voltage  independent of the load is to connect a  string of diodes wired in reverse parallel in series with one of the AC supply  leads. Here we sele...

Small-size AA cells and button cells used in electronic devices providing a terminal voltage of 1.5V are normally rated at 500 mAh. As the cells discharge, their internal impedance increases to form a potential divider along with the load and the battery terminal voltage reduces. This, in turn, reduces the performance of the gadget and we are forced to replace the battery with a new one. But the same battery can be used again in some other application that requires less current. Here’s a simple tester for quick checking of discharged pencells and button cells before throwing them away. The tester detects the holding charge of the battery and the terminal voltage to indicate whether the battery is suitable for a particular gadget or not. A 9V battery can power the circuit with sufficient voltage and current. When you close switch S1, it provides stable 6V DC to the circuit. Simple Pencell Charge Indicator Circuit Diagram The circuit uses op-amp CA3140 (IC1) as a voltage comparator. It c...

A clipping indicator is a useful accessory on any audio amplifier. It indicates when the amplifier has reached its limit and is clipping the peaks of the audio signal. In practice, quite a lot of clipping can occur before you can hear it. So why is it necessary to know when an amplifier is clipping if you can't notice it? The answer is that clipping "squares up" the waveform and square waves contain lots of higher-frequency harmonics which can easily damage the tweeters in loudspeaker systems. This circuit is a true clipping indicator as opposed to the level indicators that are commonly used in preamplifier stages. The problem with level indicators is that an amplifier's maximum output power is not constant. That's because the amplifier's supply rails are not regulated and so the maximum power available at any given instant varies, depending on the applied signal. The circuit is quite simple and is based on two BD140 PNP transistors and zener diode ZD1. Durin...

Using a few diodes and a LED, you can make a nice indicator as shown in associated schematic diagram that can be used for a lot of applications (with a bit of luck). It’s quite suitable for use in series with a doorbell or thermostat (but don’t try to use it with an electronically con-trolled central-heating boiler!). This approach allows you to make an attractive indicator for just a few pennies. The AC or DC current through the circuit causes a voltage drop across the diodes that is just enough to light the LED. As the voltage is a bit on the low side, old-fashioned red LEDs are the most suitable for this purpose. Yellow and green LEDs require a somewhat higher forward voltage, so you’ll have to first check whether it works with them. Blue and white LEDs are not suitable. You also don’t have to use modern high-efficiency types (sometimes called ‘2-mA LEDs’ or ‘3-mA LEDs’). If a DC current flows through the circuit and the LED doesn’t light up, reverse the plus and minus leads. Bridg...

This circuit not only indicates the amount of water present in the overhead tank but also gives an alarm when the tank is full. The circuit uses the widely available CD4066, bilateral switch CMOS IC to indicate the water level through LEDs. When the water is empty the wires in the tank are open circuited and the 180K resistors pulls the switch low hence opening the switch and LEDs are OFF. As the water starts filling up, first the wire in the tank connected to S1 and the + supply are shorted by water. This closes the switch S1 and turns the LED1 ON. As the water continues to fill the tank, the LEDs2 , 3 and 4 light up gradually. The no. of levels of indication can be increased to 8 if 2 CD4066 ICs are used in a similar fashion. When the water is full, the base of the transistor BC148 is pulled high by the water and this saturates the transistor, turning the buzzer ON. The SPST switch has to be opened to turn the buzzer OFF. Remember to turn the switch ON while pumping water ...