Well, my last little tip got some interest, so I thought I would post this one up.
How often do you find that you have a positive voltage signal, but wish you had a ground. Maybe you want to trigger a fan relay, or light a dash light. I like to use those little round switches with LED's, but you can't make them give a ground signal and also light up the LED in the switch. A common solution to this problem is to install an automotive ( Bosch style) relay to switch in the ground signal.
Well, here's a way I've solved this problem in the past. I was installing an electronic HVAC head unit in my El Camino, and the signal for AC Clutch engagement coming from the head unit was 12V, but extremely low amperage. I could not get it to even trigger a Micro Relay. I discovered a small transistor, designed for the automotive industry, and used in many of the modern cars- buried on a PCB somewhere in your car. The PN for the device is ZVN4206AV, and it is a MOSFET transistor. The "V" at the end of the PN indicates that it has a built in diode for protection against back current from a relay opening. If you are just using for Lights, LED's, etc, you can use the version without the "V". Available at AVNET.com, or sometimes on Ebay. Usual cost is around $.50, depending on how many you buy.
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The Transistor has three connectors- one is for your power input, one for ground, and the third goes to ground when power is sensed on the power input. The ground signal it outputs can carry a load of about .3 amps- enough for an automotive relay coil, dash light, LED, etc. In the picture above the Leg on the left is the Gnd Output, Middle leg is your 12v signal in, and the right leg is Gnd. The MOSFET has a Flat side and a round side, and as shown in the picture the flat side is down.
Here's how I used one today. I'm installing one of my little LED switches, as an emergency "Turn On" for the High Speed Coolant Fan Relay. If you only have one speed, it will work for that two. I attached 12V as normal to the switch, but then soldered the MOSFET + lead to the output lead of the switch, and the GND lead of the MOSFET to the GND tab of the switch. I then attached a GND lead to the same tab on the switch. You do not use the third Tab on the switch for the output, like you normally would. The third lead of the MOSFET is the output. This I routed to the GND side of the Fan Relay coil.
![Image]()
When you flip the switch, power is routed through the switch to the MOSFET, closing the GND connection inside the MOSFET, and thereby grounding the Third Leg. The LED light in the switch also glows. Think of it as a simple on/off switch for Ground.
How often do you find that you have a positive voltage signal, but wish you had a ground. Maybe you want to trigger a fan relay, or light a dash light. I like to use those little round switches with LED's, but you can't make them give a ground signal and also light up the LED in the switch. A common solution to this problem is to install an automotive ( Bosch style) relay to switch in the ground signal.
Well, here's a way I've solved this problem in the past. I was installing an electronic HVAC head unit in my El Camino, and the signal for AC Clutch engagement coming from the head unit was 12V, but extremely low amperage. I could not get it to even trigger a Micro Relay. I discovered a small transistor, designed for the automotive industry, and used in many of the modern cars- buried on a PCB somewhere in your car. The PN for the device is ZVN4206AV, and it is a MOSFET transistor. The "V" at the end of the PN indicates that it has a built in diode for protection against back current from a relay opening. If you are just using for Lights, LED's, etc, you can use the version without the "V". Available at AVNET.com, or sometimes on Ebay. Usual cost is around $.50, depending on how many you buy.
The Transistor has three connectors- one is for your power input, one for ground, and the third goes to ground when power is sensed on the power input. The ground signal it outputs can carry a load of about .3 amps- enough for an automotive relay coil, dash light, LED, etc. In the picture above the Leg on the left is the Gnd Output, Middle leg is your 12v signal in, and the right leg is Gnd. The MOSFET has a Flat side and a round side, and as shown in the picture the flat side is down.
Here's how I used one today. I'm installing one of my little LED switches, as an emergency "Turn On" for the High Speed Coolant Fan Relay. If you only have one speed, it will work for that two. I attached 12V as normal to the switch, but then soldered the MOSFET + lead to the output lead of the switch, and the GND lead of the MOSFET to the GND tab of the switch. I then attached a GND lead to the same tab on the switch. You do not use the third Tab on the switch for the output, like you normally would. The third lead of the MOSFET is the output. This I routed to the GND side of the Fan Relay coil.
When you flip the switch, power is routed through the switch to the MOSFET, closing the GND connection inside the MOSFET, and thereby grounding the Third Leg. The LED light in the switch also glows. Think of it as a simple on/off switch for Ground.
