What relay/switch do I use to switch based on voltage?

Question

There is one DC power source that varies in voltage (2 modes) - it is either higher than 8VDC or lower than 8VDC.
What device/relay/switch would I use to energize two separate circuits/paths based on the two voltages? i.e. circuit one would only receive power when the voltage is higher than 8vdc and circuit two would receive power only when voltage is lower.

I'm looking for a fairly cheap and easy solution!

Answer 1

Hi there.

There are a few ways to do this depending on what you are connecting to. What other supplies do you have in the circuit? How much current are you expecting in each circuit? What's the highest voltage you are expecting?



If you are looking at low currents, transistor switches may be an option. For high currents, relays (you would use a SPDT) usually are good.



To switch on the relay \ transistors a voltage detector is required to know when the voltage is above 8V. This could be an opamp comparator or a zener with buffer combination. It depends on what else is in your circuit that you can use. For the comparator you will need a constant 8V supply. Zeners won't need that supply.



EDIT:

There may be an easier way to do this (assuming I've understood all that correctly).



If you can run the airhorn off 12V you can simply just use the switch to control with no need for a voltage detector. You can wire it in parallel with the existing horn going:

Bat>Relay>fuse>newRELAY



Connect the airhorn and original horn circuits to differen sides of the newRelay. This way the newRelay will switch between which horn it is powering.



The new relay is a SPDT relay (you can get >12VDC rated automotive ones at > fuse current) and is controlled by the airhorn switch. Both switches control the relay to the battery now (if you can wire it that way).

So here are the four possible states:

- No switches on: newRelay connected to horn, Battery relay

open.



-Airhorn on: newRelay connected to airhorn, battery relay closed



-horn on: newRelay connected to horn, battery relay closed



-both on: newRelay connected to airhorn, battery relay closed



Have a look at that first to make sure it can be done with what you have. It should be fairly cheap too.

Answer 2

Use a comparator such as half an LM2903. Supply a Zener-regulated voltage to the inverting input (this needs to be low enough still to be regulated with a supply below 8V), and a suitable proportion of the unknown voltage to the non-inverting input (use a preset potentiometer so you can adjust the switching point accurately). For instance, if you use a 3V9 zener then you will need for there to be 3.9V at the + input when the supply hits 8V. Use the 2903 output to turn on a transistor and then a relay. (You will need a pull-up resistor on the output, as it's open-collector.)



Make sure that the relay you select will pull in reliably at 8V .....

Answer 3

a latching relay is gain by connecting one in each and every of its many times open contacts i sequence with the coil and then utilising a many times open pushbutton in parallel with this touch Use a 2nd touch pair on the relay to alter the availability on your load the proper layout is to latch the relay directly to grant your load (by way of a many times open touch and then change the relay off in case you get an over-modern fault this type is greater fail secure as relay mess united statesnormally lead them to instruct off the availability to the relay coil desires to be everlasting. do you ought to like the circuit to automobile latch in the time of ability up, you will possibly desire to upload a capacito and a diode to the coil circuit to initially turn the relay on

Answer 4

Battery

.|

Fuse

.|

x-------------

.|..............|

horn.....Air horn

.|..............|

K1-N.C.. K1-N.O. K1 norm closed & norm open relay throws

..\............./

....K1-C K1 pole K1 = 12 VDC SPDT RELAY

...........|

...........|

............\ steering wheel sw

...........|

.....GROUND





Battery

.....|

Fuse

.....|

.....\ Airhorn Enable Switch

.....|

K1-COIL

.....|

GROUND