| Electronics > Beginners |
| Help me create Low Voltage switching relay/circuit |
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| JS:
LED voltage range is quite small, you can take the most out of your battery keeping the losses low using a suitable current source, for small LEDs a J-FET might do, but you need something to keep the LEDs in the happy-working area. To switch between the two batteries a relay could do, but you need some power to keep the relay on, which could be draining your battery noticeable faster than the LEDs. As you will have your LEDs with a CC source you could use them as reference, then switching between two mosfets but that would require a comparator. That would be the only IC in that case, but to ensure start up the preferred state for the mosfet should make them connected and the unconnected should be done by the comparator, for the unused battery. Then you might want to protect the second battery from under-voltage as well, which makes things a bit harder. The LED current would change a lot things for this project. One option might be a miss used CMOS logic IC, using the rails as output and the outputs as rails, check for a controlled rectifier using one chip like that... pretty cool way to solve this sort of problems! JS |
| Brumby:
--- Quote from: THATguy on September 09, 2018, 03:07:03 am ---Just a few volts and very short distance. A switching delay would be fine, keeping the current in the circuit is not critical. Load is just a LED. That is the sole purpose, a torch with a back up battery. --- End quote --- This is a start. We now know portability will be an issue - and that the power consumption of any solution should be as low as possible. But we still don't know the actual voltage - nor the light output (and, hence, power consumption) of the LED. We also don't have much of a handle on physical size constraints. The phrase "just a LED" is not as helpful as you might think. Is this LED for a pocket sized torch or one of those that can light up someone on the other side of a football field? You haven't made this clear at all. |
| THATguy:
Okay using an LDR could be a way to switch over to the fresh battery. The second picture shows how the LDR(C) & LED(B) would be setup physically. I've not shown the other side of the battery circuit - but it would be a mirror of this side. The second diode is there because just after this is where the other battery would join. There are a few issues with this: When this battery switches on the LDR would activate again switching it offf - when it switches on there needs to be a way of disabling the LDR... sigh it's already getting messy & too compkex. |
| Zero999:
--- Quote from: THATguy on September 23, 2018, 06:21:25 pm ---Okay using an LDR could be a way to switch over to the fresh battery. The second picture shows how the LDR(C) & LED(B) would be setup physically. I've not shown the other side of the battery circuit - but it would be a mirror of this side. The second diode is there because just after this is where the other battery would join. There are a few issues with this: When this battery switches on the LDR would activate again switching it offf - when it switches on there needs to be a way of disabling the LDR... sigh it's already getting messy & too compkex. --- End quote --- The transistor's base is shorted to the collector, which is connected across the battery, causing a large current to flow through the transistor, resulting in its destruction. |
| THATguy:
Yeah, I might need some resistors, its already getting too big & messy (resistors are wasteful in a low voltage circuit). Don't suppose there are light emitting resistors? :D Someone has suggested I look at Triac, Thyristor, solid state relay, & analog switches, though analog switches have some specific voltage requirements. Another good suggestion I've had is to look up Low Battery Backup circuits Most of them seem overly complex. This one looks simple enough, what do you think? --- End quote --- strange circuit Not really sure why the battery in this one wouldn't just drain - can't see why there is even a resistor pathway either? |
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