| Electronics > Beginners |
| Low battery warning circuit |
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| @rt:
Hi :) Is there a need for any more than a simple resistor divider in this situation? I only have a digital input, and Vcc for the shift register is 5V. In the real circuit the battery could be up to 13.8 Volts, so I realise the 150K resistor should be a higher value to prevent the shift register pin ever seeing a voltage above it’s 5V supply rail. This way I think diode clamping the shift register input can be avoided. Software ensures this input must read the warning status 253 times consecutively before the status is actually acted on. It has been suggested that I use the BC548 level shifter on this page, but I fail to see the point: https://jeelabs.org/book/1504d/ The 12V and logic would be swapped to the other direction in this case. |
| digsys:
Apart from the issues you mention - your circuit doesn't take into account - Varying battery voltage - I'm assuming you are working directly off a battery ie 11.5V-ish to 13.8V-ish, so your reference will vary a lot. Potentiometers drift a little on top of that. Check out the TPS3700 - has 2 in-built UV and OV precision references, good to 18V, O/C output = ok for ANY logic output / input, cheap and easy. I use them for all battery / voltage monitoring. Only require 2-3 resistors. The suggested circuits need no extra work. |
| @rt:
I don’t quite understand. The 5V output of the regulator that is powering the shift register is the stable voltage reference. Therefore, I expect the high/low logic threshold voltage for its input to be relatively stable. The battery voltage is supposed to vary. I maybe see a problem if the battery was so low that the regulator couldn’t supply 5V, but the idea was to catch the condition before that happens. |
| digsys:
OK a bit lost here. If you only need to know when the battery voltage drops to ~8.0V (assuming ~3V loss over the 5.0V regulators), then that will work. I am assuming you are using a SLA or other type of Lead acid? in which case you need to protect that MORE than just the logic side. That is why I suggested that part. SLAs should never go OV / UV. Is this correct? At the very least, I'd put a 4v7 400mW zener on the wiper of the pot. |
| @rt:
It will never be SLA. It’s 8xAA in series for nominal 12V, So I realise the voltage will begin higher than 12V with fresh batteries (making the 150K resistor too low a value). In the end this will be 3x18650 batteries (nominal 10.8V), so discharging those too far will be an issue, but that should just mean the 150K resistor needs to be adjusted yet again. Understood about the zener. |
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