razor02097
08-05-2010, 01:08 PM
In the world of electronic controllers nothing is as versatile and necessary as a latch circuit. A latch circuit is a way to turn something on and off using electronic pulses.
Simply explained let us say you have an older car with a regular radio. When you start the car, the radio comes on. When you turn the car off, the radio turns off. Let us say that you wished when you turned the car off the radio remains on until you get out of the car. (much the same as the microcontroller BCM does in newer cars) How could you go about doing this?
Option 1 is to wire a switch to manually turn the radio on and off. This means you would have to remember to flip the switch every time you left the car or risk running your battery down.
Option 2 is to build and install a latch circuit. This option is nice because it automates the task, allowing the user to "forget" about the task.
Black box theory: This explains the inputs and outputs of the circuit. The actual circuit is referred to as the "black box"
You have 4 inputs and 1 output.
Inputs
+12V constant power
Vehicle Ground
Trigger wire to turn circuit on
Trigger wire to turn circuit off
Output
+12V or Ground out
Circuit explained: The following diagram uses 2 relays. A load relay which powers your device and a controller relay which controls whether your circuit is on or off.
The load relay can be the typical automotive 30A 4 pin relay.
The controller relay needs to be a 5 pin double pole, single throw or DPST relay.
The following schematic shows how to wire them to provide a 12V output with 12V trigger wires. The load is the radio example explained above.
Technical note: There are 3 diodes necessary to prevent circuit failure on this circuit . Each relay should have a protection diode (http://%5c%22http//tinyurl.com/2d2t5sr%5C%22). The other diode should be installed inline with the "on" trigger wire. In the following schematic I show where you put the diodes and how to orient them (http://upload.wikimedia.org/wikipedia/commons/0/05/Diode_3D_and_ckt.png).
4480
When you turn the car on it will power up the radio and set the latch circuit. The relay remains on even when the ignition is switched off, this is because the relay basically provides a feedback loop to itself. To turn it off we use the controller relay to break it's ground connection. When the interior lights power the controller relay the ground connection is broken turning the load relay off.
This circuit can be used for anything from radio, entertainment to home made turbo timers and light controllers. I hope a few of you will try out this circuit and find new uses for it.
Edit: The following are quick links job specific circuits.
latch circuit to operate 2 stage controller (http://ltxtech.com/forums/showthread.php?12838-Relay-latch-circuit-what-it-is-and-how-to-build-one.&p=163435&viewfull=1#post163435)
Simply explained let us say you have an older car with a regular radio. When you start the car, the radio comes on. When you turn the car off, the radio turns off. Let us say that you wished when you turned the car off the radio remains on until you get out of the car. (much the same as the microcontroller BCM does in newer cars) How could you go about doing this?
Option 1 is to wire a switch to manually turn the radio on and off. This means you would have to remember to flip the switch every time you left the car or risk running your battery down.
Option 2 is to build and install a latch circuit. This option is nice because it automates the task, allowing the user to "forget" about the task.
Black box theory: This explains the inputs and outputs of the circuit. The actual circuit is referred to as the "black box"
You have 4 inputs and 1 output.
Inputs
+12V constant power
Vehicle Ground
Trigger wire to turn circuit on
Trigger wire to turn circuit off
Output
+12V or Ground out
Circuit explained: The following diagram uses 2 relays. A load relay which powers your device and a controller relay which controls whether your circuit is on or off.
The load relay can be the typical automotive 30A 4 pin relay.
The controller relay needs to be a 5 pin double pole, single throw or DPST relay.
The following schematic shows how to wire them to provide a 12V output with 12V trigger wires. The load is the radio example explained above.
Technical note: There are 3 diodes necessary to prevent circuit failure on this circuit . Each relay should have a protection diode (http://%5c%22http//tinyurl.com/2d2t5sr%5C%22). The other diode should be installed inline with the "on" trigger wire. In the following schematic I show where you put the diodes and how to orient them (http://upload.wikimedia.org/wikipedia/commons/0/05/Diode_3D_and_ckt.png).
4480
When you turn the car on it will power up the radio and set the latch circuit. The relay remains on even when the ignition is switched off, this is because the relay basically provides a feedback loop to itself. To turn it off we use the controller relay to break it's ground connection. When the interior lights power the controller relay the ground connection is broken turning the load relay off.
This circuit can be used for anything from radio, entertainment to home made turbo timers and light controllers. I hope a few of you will try out this circuit and find new uses for it.
Edit: The following are quick links job specific circuits.
latch circuit to operate 2 stage controller (http://ltxtech.com/forums/showthread.php?12838-Relay-latch-circuit-what-it-is-and-how-to-build-one.&p=163435&viewfull=1#post163435)