Unpowered circuit timer - Page 1
Forum section: electronics / projects

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 21, 2014, 10:24:23 am
content: hi,I been asked to design a circuit that do some stuff and capable of detecting if the circuit been off power for a certain period ( 15 min app.) and can't use any backup battery to run any kind of timer, so circuit must be totally unpowered. I came up with the circuit linked here where a capacitor is charged once the uC finished a starting sequence, but before that the adc will measure voltage left on it and give an approximative time. well been simulated and with no surprise it work and been tested and again with no surprise precision is a disaster. So any idea for a better precise solution or any suggestion to guaranties a better precision and stability overtime of this actual circuit.

author: sacherjj
author_rank: Frequent Contributor
timestamp: July 21, 2014, 12:01:54 pm
content: To get suggestions we need two things.1 - Maximum and minimum circuit off time.2 - Accuracy requiredYou can't use a backup battery, but can you use a backup capacitor? I would think an RTC with backup cap that is charged when powered would be pretty accurate.

author: leppie
author_rank: Frequent Contributor
timestamp: July 21, 2014, 12:11:49 pm
content: ADC sounds like overkill for something that should be digital.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2014, 12:14:34 pm
content: Eliminate the discharge path through C-B of Q1, R9 and R10 to the +5V, which will presumably be closer to 0V when the circuit is off.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 21, 2014, 12:25:29 pm
content: Well 15 min to 30 min off time detection capability, accuracy +/- 2 min. sure thing i can use capacitor , but i think to run an RTC it will need to be a large capacitor ? reliability overtime is an issue too so i prefer not to use electrolyte capacitor, this thing can be installed in -20°C environment. The capacitor in the schematic even if i draw it as polarised one ,i will use a ceramic one, better precision and stability. I discarded the idea of using any kind of timing circuit once i was informed not to use any battery, but i will give RTC with a cap a try. thanks

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 21, 2014, 12:34:03 pm
content: Quote from: Andy Watson on July 21, 2014, 12:14:34 pmOkay , and suggestion for the amp , should i worry about leakage current through input when it's unpowered, any suggestion to use a specific type of amp.

author: sacherjj
author_rank: Frequent Contributor
timestamp: July 21, 2014, 12:42:19 pm
content: Trickle charge capable RTCs with 0.2F super cap will run on the order of 1 week. I doubt you will have an issue with keeping the RTC running for 30 minutes. You will be cheaper if you can get an analog solution accurate enough, but rolling an RTC would be pretty straight forward.Edit:http://www.digikey.com/product-detail/en/AS1801-BTDT/AS1801-BTDTCT-ND/3095257Looking at this data sheet, 0.2F is 267h. So you won't need much of a backup cap.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 21, 2014, 01:31:47 pm
content: @sacherjj, at this point cost is not a problem, never heard of "Trickle charge capable RTCs" now i searched for some reference and the one you just mentioned and am giving up the capacitor discharge solution, will order some of those parts to play with, thanks man

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 05:25:57 pm
content: Quote from: hamdi.tn on July 21, 2014, 12:34:03 pmWhen the operational amplifier is not powered, then the ESD protection diode from the input to the positive supply is going to discharge the capacitor into the positive supply.