simple switch off time delay schematic

Posted on Feb 5, 2014

The two circuits di atas illustrate opening a relay contact a short time after the ignition or ligh switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to 12 volts. The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor i

Click here to download the full size of the above Circuit.

s not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12. 5 volt input. The common emitter configuration on the right offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out voltages less of a concern but requires an extra resistor in series with transistor base. The common emitter (circuit on the right) is the better circuit since the series base resistor can be selected to obtain the desired delay time whereas the capacitor must be selected for the common collector (or an additional resistor used in parallel with the capacitor). The time delay for the common emitter will be approximately 3 time constants or 3*R*C. The capacitor/resistor values can be worked out from the relay coil current and transistor gain. For example a 120 ohm relay coil will draw 100 mA at 12 volts and assumming a transistor gain of 30, the base current will be 100/30 = 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1. 4 = 10. 6. The resistor value will be the voltage/current = 10. 6/0. 003 = 3533 or about 3. 6K. The capacitor value for a 15 second delay will be 15/3R = 1327 uF. We can use a standard 1000 uF capacitor and increase the resistor proportionally to get 15 seconds.

Visit page

Leave Comment

characters left:

Related Circuits

Color sensor

Color sensor. Colour sensor is an interesting project for hobbyists. The cir- cuit can sense eight colours, i.e. blue, green and red (primary colours); magenta, yellow.

Dew sensor

Dew sensor. Dew (condensed moisture) ad- versely affects the normal per- formance of sensitive electronic devices. A low-cost circuit described here can be used to.

Audio Visual Ringer

Audio Visual Ringer. Many a times one needs an extra telephone ringer in an ad-joining room to know if there is an incoming call. For example, if the telephone is installed in.

Multipurpose Circuit for telephone

Multipurpose Circuit for telephone. This add-on device for telephones can be connected in parallel to the telephone instrument. The circuit provides audio-visual indication of on-hook,.

Off line Telephone tester

Off line Telephone tester. Here is a circuit of an off-line telephone tester which does not require any telephone line for testing a telephone instrument. The circuit is so simple that.

Telephone Headgear

Telephone Headgear. Acompact, inexpensive and low component count telecom head- set can be constructed using two readily available transistors and a few other electronic.

Telephone line based audio muting and light on circuit

Telephone line based audio muting and light on circuit. Very often when enjoying music or watching TV at high audio level, we may not be able to hear a telephone ring and thus miss an important...

Telephone Ringer using 556 dual timers

Telephone Ringer using 556 dual timers. Using modulated rectangular waves of different time periods, The circuit presented here produces ringing tones similar to those produced by a telephone. The.

New Circuits

Zero consumption Anti-theft High power Code Switch for E-Bike Batteries

This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
Ultrasonic distance finder circuit

The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Simple Metal detector circuit with CD4030

Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Fully automatic watering circuit for flower pots

Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
Simple deep searching metal detector circuit

The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
Sub-Harmonic Bass Converter for Electric Guitars

The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
Echo Depth Sounding Sonar for Boats

In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
100W HiFi Audio Amplifier Circuit

This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
Plant-Pot Water Level Indicator Circuit

A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....

Create NOW your QR MENU for your Restaurant / Hotel / Bar / Caffe,

Comprehensive ordering/paying features. fully personalized to suit your Business.

Update Anytime! Easy and Simple!

Popular Circuits

Leach amplifier

A hot topic of amplifier design in the 1970s was "transient intermodulation distortion" (TIM)....

Nickel Metal Hydride ( NiMH ) Charger

This circuit will charge a 12v battery (10 NiMH cells) @ approx 220mA. It is a very simple...

Microprocessor programmable interval timer

The microprocessor sends out an 8-bit binary code on its 8-bit I/O bus (the binary value needed...

Solid-state-light-dissolver

The dimming action is controlled by varying the amount of current passed through triac Q4 and,...

Video-gain-block

This configuration utilizes the wide bandwidth aod speed of HA-2540. plus the output capability...

1296Mhz Rf Amplifier

Using an NEC70083, this 1296-MHz amplifier delivers about 17-dB gain and around 1- to 1.5-dB...

car tracking device circuit diagram

Tank circuit C2 and L1 is used to tune the transmitter. The antenna is coupled to the transmitter...

potentiometer Making an adjustable resistance

Actually this part of the circuit is isolated from any other component (except via 5v & ground)....

switches Raspberry Pi GPIO

Experiments with couple of floppy drives I have and it appears that on the step pin, current is...

50hz accurate oscillator circuit

The IC ELM446 is an 8 pin digital divider integrated circuit, that provides both 50Hz and 1Hz...

Schematic Diagram LM3909 IC Color organ electronic project circuit

Using LM 3909 LED flasher IC, can be designed various electronic projects. As you can see in this...

TB6600 5A Single Axis 5A CNC Stepper Driver For CNC Milling Machine DIY Max 50V

Two-phase hybrid stepping motor dynamic voltage 12 to 48V, the current is less than 5A outer...

Basic RF Oscillator

Make a basic RF oscillator. The circuit is ridiculously simple, so I will skip all the...

AC welder saving controller circuit diagram

AC welder is intermittent operation, the intermittent load power consumption during the period up...