Telephone Headgear -  A compact, inexpensive and low component count telecom headset can be constructed using two readily available transistors and a few other electronic components. Since the circuit draws very little current, it is ideal for parallel operation with electronic Telephone set. . . 

Telephone In Use Indicator -  This circuit will illuminate a LED if one of your telephones is in use. It should work in all countries (Including UK) that have a standing line voltage above 48 Volts DC. . . .  [Miroslav Adzic]

Telephone in Use LED Indicator -  . . .  [Bill Bowden, designer]

Telephone in Use Light -  ASCII format . . . 

Telephone In-Use LED Indicator -  Circuit to illuminate an LED when any telephone extension is off the hook. . . .  [Bill Bowden's circuit]

Telephone Lamp Circuit -  Here is a White LED Lamp that turns on when the telephone rings. The cool white light helps to locate the phone in darkness and also helps to note the message or telephone number. The lamp remains on for three minutes then turns off. The telephone lamp circuit is powered by a 9 volt PP3 battery and connected to the telephone. . .  [Circuit Posted by D Mohankumar]

Telephone Line Based Audio Muting and Light on Circuit -  This circuit mutes television audio/ music system when a telephone ring arrives and lights up a bulb to indicate the telephone ring. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Telephone Line In-Use Indicator -  This circuit gives a visual indication of whether a telephone line is in use or not, helping two or more people sharing the line to avoid interrupting each other's calls. As the circuit connects in parallel with the telephone line, it can also detect phone tapping/misuse attempts. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Telephone Off-Hook Indicator -  This circuit provides a steady logical "high" indication when the telephone is off its hook. Operation is as follows:During the off-hook condition, DC telephone line voltage flows through clamping diodes CR1 and CR2, which develop about 1.5volts across resistor R1. This voltage is provided to the input of optical coupler U1 via current-limiting resistor R2, turning on U1's internal light-emitting diode. Under these conditions, U1's NPN output transistor turns on, providing the +5 to +15 volt logic "high" level at its emitter, pin 4. Pin 4 of the coupler may be connected to subsequent indicator circuitry, such as a transistor-controlled lamp or audible device. . . .  [Designed by Arthur Harrison]

Telephone Status Indicator -  "Hello". . . meet BUSYER-7Z. The BUSYER-7Z is an educational telephone line busy indicator kit. It lets you know if someone else is on the phone, line in-use (line is off-hook) before you picking up. Large bright LED will indicate the phone line in-use status. No more “Ops, ” “I’m sorry, ” “I didn’t mean to interrupt, ” “I’m on the phone. ” The BUSYER-7Z will light when the phone line is off-Hook. It will ends interruptions by indicating that you are “On the phone. ”. . .  [Kit from Elich Industrial Development, Corp]

Telephone Status Indicator for 4 Lines -  Many home businesses use multiple phone lines. This circuit gives you a visual indication when a line is in operation. The two AA battery cells should provide enough power for about one year of operation . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/10/00]

Temperature Indicator -  This circuit sets off a buzzer after the last LED (led4) is lit. It is a simple but fun circuit to experiment with. You can replace the buzzer with something else, add a trimpot to adjust sensitivity, etc, etc. The possibilities are endles. . .  [Tony van Roon's circuit]

Temperature Indicator-CUM-Controller -  Here is an easy-to-construct temperature indicator-cum-controller that can be interfaced with a heater’s coil to maintain the ambient room temperature. The controller is based on Atmega8535 microcontroller, which makes it dynamic and faster, and uses an LCD module to display and two keys to increase or decrease the set values. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Three-Volt Low Battery Voltage Flasher -  Many battery powered devices use two AA alkaline cells. Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating. The hobby circuit below can be connected to a 3v battery, to give you some . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/01/98]

Transients Indicator Circuit -  Instant voltage transients are common in the power lines especially in places where machinery is operating. Inductive loads and high power transformers creates transients and spikes in the mains lines. Such voltage fluctuations are dangerous to SMPS based power supply and Microcontrollers. Here is a simple circuit that. . .  [Circuit Posted by D Mohankumar]

Transmit to a Remote LED Indictor Light -  There are times when you would like to transmit a signal from one LED indicator light to second LED at another location. The circuit below works well for this application. It takes advantage of the fact that the internal infrared LED inside an opto-isolator . . . [Hobby Circuit designed by David A. Johnson P.E.,  12/28/08]

Tripler Converts 5 Volt to 15V -  AN-726 Analog Devices Application Note. . .  [Maxim Integrated]

Two Line Telephone Line Status Indicator -  Many home businesses use multiple phone lines. This circuit gives you a visual indication when a line is in operation. The two AA battery cells should provide enough power for about one year of operation. The circuit is line polarity insensitive . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/10/00]

Two Transistor Oscillators -  This page has two unusual two-transistor oscillators.   I set the component values for a low frequency application.   Both circuits draw only about 1 microamp of current. … [Circuit designed by David A. Johnson P. E. , 9/16/07] . . . [Circuit by David Johnson P.E.,  12/14/02]

Two-Transistor Low Power Oscillators -  This page has two unusual two-transistor oscillators. I set the component values for a low frequency application. Both circuits draw only about 1 micro Amp of current . . . [Hobby Circuit designed by Dave Johnson P.E.,  12/14/02]

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