Low battery indicator
Can someone please provide me with a schematic to turn on an LED when a 9v PP3 gets low (~<20%), no microcontrollers, the simpler the better. I am hoping you can do this with a few diodes, transistors, resistors, and maybe some capacitors, but I can't think how this would work.
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Have you tried using an IC like the following for this? MC34064P 4.5v undervoltage Sensor
Feb 13, 2011. 1:43 AMandy70707 (author)
says:
A 3-legged IC like that would be good, as I previously said, an 8-pin DIP would be too large for my application and would probably have to use flying wires as I am using stripboard, although it only does 4.6v. By then, the voltage regulator would have stopped drawing any power and the LED probably wouldn't even be on. The voltage regulator stops working at about 7v, so I want the low battery indicator to trigger around 7.8v. A 9v battery is considered 'dead' at around 7.2v anyway. As I said, I have already solved the problem using a transistor, 3 resistors and a potentiometer.
Conceptually, I'd start with a joule thief circuit, so that the LED will work when the battery is low. Then, you add in a small relay in the normally closed (NC) configuration. You want a relay where the "minimum trigger voltage" is either adjustable, or is fixed at the value you want as a cutoff (e.g., 7.2V).
The idea is that as long as the battery is strong enough, it holds the relay open with some small power drain. Once the battery drops too low, the relay closes, and the joule thief kicks in to illuminate the LED.
The idea is that as long as the battery is strong enough, it holds the relay open with some small power drain. Once the battery drops too low, the relay closes, and the joule thief kicks in to illuminate the LED.
Feb 8, 2011. 8:24 AMacmefixer
says:
A typical 9V relay uses more current than a 9V battery can supply. It alone would run the battery down. The Joule Thief is not needed, and would not work at 9V. Here is one circuit that should work.
Feb 11, 2011. 9:06 AMandy70707 (author)
says:
I just breadboarded the circuit, and although it works in my emulation software, it doesn't work in real life. My transistors and resistors are all correct, but the LED is permanently on for both a 10v battery (full) and 7.2v (empty) 9v PP3. It does seem to dim slightly for the empty battery, but It should be on if it is low and off if it is anything about about 7.8v. Adjusting the 100k pot also has absolutely no effect.
Feb 11, 2011. 1:01 PMacmefixer
says:
R3 should be zero ohms or a short or jumper. Measure the voltage at Q1 base to minus. You should be able to adjust R2 and the voltage should go from 0 to a bit more than a half volt. When it gets up to a half volt Q1 should start to conduct and the LED should go out. If you measure less than a half volt, your R1 resistor and/or R2 pot may be the wrong value or out of tolerance. These determine the set point. If you remove the jumper and make R3 equal to 47k, then the pot should be able to adjust so that the voltage measured at Q1 base is a half volt. If you are getting a half volt or more at Q1 base and the LED does not go out, then you may have a problem with Q1, Q2, etc.
One quick check you can make is to put a short across Q1, E to C, and the LED should go out.
One of the first things I would check is if the Q1 and Q2 are correctly inserted. If you are using 2N3904 or 2N4401 for Q1 and Q2 then the pinout is E B C. This is the opposite of transistors such as BC547 or BC337. Also, I've taken used transistors from other projects and then used them, and found that they were damaged earlier. Use some transistors that you know are new or not damaged.
One quick check you can make is to put a short across Q1, E to C, and the LED should go out.
One of the first things I would check is if the Q1 and Q2 are correctly inserted. If you are using 2N3904 or 2N4401 for Q1 and Q2 then the pinout is E B C. This is the opposite of transistors such as BC547 or BC337. Also, I've taken used transistors from other projects and then used them, and found that they were damaged earlier. Use some transistors that you know are new or not damaged.
Feb 11, 2011. 1:22 PMandy70707 (author)
says:
I used a jumper for R3, and measuring Q1, I got 280mv-680mv when adjusting the pot, although the LED remains unaffected. I am using standard 1/4 watt, 5% tolerance resistors, all the exact value from the diagram except R5 I changed to 1.5k instead of 10 as when emulating it, 10k was too strong for it to light (even with 1.5k, it is still fairly dim). Shorting Q1 does make the LED go out, and they are 2N3904 diodes, all brand new.
Feb 12, 2011. 10:57 AMacmefixer
says:
I assembled the circuit this morning, and it worked okay. You can read the details in my blog about it. I noticed that it is sensitive to AC hum. I was using a pot with a metal shaft and when I touched the shaft the LED glowed. The input is very sensitive to AC. The capacitor I suggested should be between the base and emitter of Q1. A .01 to 0.1 uF should work. Also, if you are using a breadboard or protoboard, the board itself may be causing the circuit to pick up hum. If the board has a metal plate, ground it to the negative. But once you put the cap in, it should no longer have this problem.
Feb 12, 2011. 3:03 PMandy70707 (author)
says:
Thanks for your help, I still couldn't figure out the problem with that circuit, but I built a similar one using only 1 transistor and it works, although I had a few problems, but changing the transistor seemed to work. I tested both transistors with a multimeter and they work fine, although only 1 would work in the circuit. They are both 2N3904 transistors, but made by different companies, so maybe the first two are just bad (they were free anyway).
Feb 12, 2011. 4:25 PMacmefixer
says:
When I have had this stubborn problem, I set the circuit aside for a few days to clear my head, because I usually have been overlooking the same problem again and again. Sometimes when I come back and find the problem I say to myself why didn't I think of that before? If not, then I pull the whole circuit apart, clip the leads off, and dispose of it, so I won't have any chance of reusing a bad part. A few cents in parts it not worth hours of frustration.
Like I said in the blog, the circuit is sensitive to hum. The problem is taken care of with the 0.1 uF from base to emitter Q1.
Mine works good, the only thing I had to do was change R3 from zero to 22k to move the setting away from the end of the pot.
Like I said in the blog, the circuit is sensitive to hum. The problem is taken care of with the 0.1 uF from base to emitter Q1.
Mine works good, the only thing I had to do was change R3 from zero to 22k to move the setting away from the end of the pot.
Feb 11, 2011. 2:17 PMacmefixer
says:
I gave 10k for R5 because it will reduce the drain on the battery. If you use a superbright red LED it should help a lot.
When the voltage at the base of Q1 gets up to .5 or more volts, it should turn on and the collector voltage should drop below a half volt. That should turn off Q2.
One other possible cause of problems might be oscillations. If you put a capacitor, 0.01 to 0.1 uF across Q1, collector to emitter, it should damp out oscillations. Also, your battery or power supply should have a 10uF or more capacitor across it.
When the voltage at the base of Q1 gets up to .5 or more volts, it should turn on and the collector voltage should drop below a half volt. That should turn off Q2.
One other possible cause of problems might be oscillations. If you put a capacitor, 0.01 to 0.1 uF across Q1, collector to emitter, it should damp out oscillations. Also, your battery or power supply should have a 10uF or more capacitor across it.
Feb 11, 2011. 4:58 PMandy70707 (author)
says:
I will try adjusting R3, and as for R5, I am not too bothered about battery drain as LEDs are only 25ma, and it will basically indicate that the user needs to replace the battery as the voltage is too low to charge, so it can use the remainder of the battery without any problems. I measured Q1 with an oscilloscope and is appears to be free from any kind of interference. As for Goodhart, I meant to say transistor, not diode. Sorry for the confusion.
Feb 8, 2011. 11:15 AMandy70707 (author)
says:
Thanks, that looks pretty good. I had a play around with it in some circuit emulation software, and at the moment it seems to do the opposite (dim the LED when battery is low), but hopefully I will be able to get it to work in reverse. Its a good start, so thanks!
Feb 8, 2011. 12:52 PMacmefixer
says:
If you find it dims when the battery gets low, then you haven't got the right values for the R3 and R2 potentiometer. When they are set correctly, the first transistor is turned on when the voltage is 7.5V or more, and this shunts to ground the current to the base of the second transistor, keeping it off.
When the voltage drops below 7V, the first transistor turns off, the base of the second transistor gets current from the 100k R4, and the collector lets the current from the LED go to ground. The LED then lights.
I gave the value of R4 as 100k, but it should be higher, 470k should work and should save some battery current. The R3 should be set to zero for starters, and only increased if needed.
Feb 7, 2011. 3:35 PMandy70707 (author)
says:
Thanks, I never really thought about that. Is there a way to do this with semiconductors instead as it is going to be part of a phone charger circuit and relays are large and a bit primitive.
There are solid-state relays, which are neither large nor primitive :-) Charging cell-phone and other "smart" batteries is more complicated, as they have some built-in electronics of their own. You may need to do a bit more research (have you tried searching for "battery charger circuits" online?) to get up to speed on some of the details.
Feb 7, 2011. 11:05 PMandy70707 (author)
says:
I had a look at at solid state relays, but the trigger voltage is too high and they are quite expensive. I was hoping to do it with transistors. As fir the charger circuit, I will not be charging it directly, but I have a simple voltage regulator to go from 9v to 5v (USB).
Feb 8, 2011. 8:24 PMguyfrom7up
says:
This can be accomplished by using a basic opamp and a zener diode, such as:
http://www.aaroncake.net/circuits/lowvolt.asp
http://www.aaroncake.net/circuits/lowvolt.asp
Feb 9, 2011. 1:42 PMguyfrom7up
says:
haha hey! yeah I've been busy and stuff and I just stop by once in a while.
AP Classes suck, lol
AP Classes suck, lol
Well, it's certainly good to know you are still around :-)
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