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Topic: Need 24 channel isolated ADC inputs? (Read 3 times) previous topic - next topic

dc42

#5
Sep 01, 2013, 02:48 pm Last Edit: Sep 01, 2013, 02:50 pm by dc42 Reason: 1
Here are two solutions:

1. Use a bank of DPST reed relays such as http://uk.mouser.com/ProductDetail/Coto-Technology/8002-05-10/?qs=sGAEpiMZZMv4tz1TW%2fArE7b2edLOV1FE%2fukC5Xbo4CI%3d to connect a capacitor across one of the cells at a time. See RL1, RL2 ... in the schematic. The series resistors protect against battery shorting and limit the capacitor charge current. Then use another DPST reed relay (RL0) to connect the capacitor to ground and an analog input pin. Drive all the relays from 5 Arduino pins using 3 x 74HC138 demultiplexer chips (this guarantees that you only activate one of them at a time). Don't forget the flyback diodes.

2. Use one ATtiny25 processor per cell, powered from the cell. Each one measures its own Vcc using its internal 1.1V analog reference. Calibrate the 1.1V reference of each ATtiny and store the calibration constant in EEPROM. Program the ATtinys to remain in sleep mode until woken up to measure the cell voltage. Use opto isolators to communicate between the Arduino and the ATtinys. You can either connect them in a ring configuration (needs 25 opto isolators, but if a single ATtiny unit or single cell fails then you get no readings at all), or you can connect them in a parallel bus configuration (needs 48 opto isolators).

The costs should be about the same, since one DPST reed relay + 2 resistors + a diode costs about as much as one ATtiny + 1 or 2 opto isolators + one resistor (to drive the opto isolator) + one decoupling capacitor; but the first suggestion is probably easier to get working.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

mightyboyev

Thanks for the replies,  I will look closer at the Atiny25 idea.... I am not a big fan of a relay style solution..... only a personal thing. Any optos that you would recomend looking into as well?
Cheers

dc42

Just about any opto isolator will do for this, provided it has a reasonable current transfer ratio at low currents. The cheapest on that my usual supplier has is http://uk.farnell.com/everlight/el1112-ta-vg/optocoupler-tr-o-p-sop-5/dp/1852690, which looks just fine, assuming you will be making a pcb and are happy to use SMD components.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

michinyon

what is that "power on failsafe"  relay expected to do, exactly ?

CrossRoads

Make sure no relay turns on until commanded. It keeps the individual coils in the off state until the sketch starts up and makes the overall GND connection so they can be turned on one by one.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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