Charge/Low Voltage Sensing Circuit: Will This Fry a Pin? Do I need the schmitt?

Zoot

I cribbed this charge-sensing circuit from an old (~70s) book on electronics and robots, and since I had the parts, I wired it up.

It works perfectly -- I get a logic 0/1 on PIN when the charger is hooked up to my bot, and I've bench tested the short-circuit diode (D1) and fuse protection.

However, before I hook it up to an actual pin on my Stamp, I want to know if it's OK. I'm a little concerned that the R2 pull-up should be a higher value? and maybe I should put in a 220 protection resistor between the circuit and the Stamp pin? Any other possible problems? I'm still pretty much a newbie at circuit design.

I'm using this because eventually I'm going to have my bot seek out it's charger and then it needs to know that it's hooked up. I use a trickle charger and when the current drops to, well, a trickle, the output of this circuit goes back to logic 0. There are charging prongs on the front of the bot, hence the need for short-circuit protection across those terminals.

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When the going gets weird, the weird turn pro. -- HST

Post Edited (Zoot) : 10/4/2006 10:51:26 PM GMT

Mike Green

The Zener diode and series 470 ohm resistor is already a pretty good protection system. It's always better to put a resistor between that point and the Stamp pin. I'd suggest 10K since you're only voltage sensing. No significant current has to flow through it

Zoot

When you mean voltage sensing, you mean that I'm just sensing the 1/0 logic voltage at my Stamp PIN, right? -- the circuit is supposedly checking whether or not significant chage current is coming across the charging terminals.

I'll plug it into a pin with 10K resistor and try it out. Thanks!

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When the going gets weird, the weird turn pro. -- HST

Zoot

OK -- I breadboarded and then transferred to a protoboard the circuit attached below EXCEPT FOR the Schmitt inverter, which I'll be putting right next to the pins of my BS2p40. I tested the circuit with 5V, 12V and charger power supply and it works great, and I get good logic levels when the "battery" voltage drops below a preset 10.9v and when >= 12V is coming in to the charger terminals.

I have a few questions because I realized that while the circuit works great, I don't understand everything about it.

- do I need the Schmitt inverter that goes to my low voltage pin if I account for invalid logic levels in software (i.e., a kind of "debounce" or count on the pin before declaring that a real low voltage state on my battery exists)?

- if I omit the schmitt inverter, would I need to put a limiting resistor between the Norton IC and my Stamp pin?

- related -- if you use the output of a Schmitt, do you never need a limiting resistor between the inverter output and a Stamp pin? I guess what I'm asking is if you had bad voltage/current on the input to the Schmitt, I guess you could fry the Schmitt, but you wouldn't fry the Stamp pin?

Any commentary or advice is greatly appreciated in advance.

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When the going gets weird, the weird turn pro. -- HST

terahertz

Without using a simulator or anything, I read this circuit like this. Assuming the battery stays at 12V and (-charger) stays grounded then there are ony 2^2 = 4 options:

1)···if· charger··=·· < 12.7V·····and · PIN= 5V····then· PIN·· outflows 5/470 amps and no current flows into battery



2)·· if charger =· >12.7V····· and·· PIN = 5V·· then·· PIN· doesn't source or sink current and there is unknown·current flow into battery from (+CHARGER)



3)·· if charger = < 12.7V··· and·· PIN = 0V·· then· nothing happens



4)·· if charger = > 12.7V and PIN = 0V·· then· PIN·· has a miniscule amount of sinked current·· ZD1 has exceeded breakdown voltage and is conducting

Zoot

Not sure if I was clear (probably not) -- PIN 13 and PIN 14 are always inputs. I read PIN 14 to see if battery voltage is low, so the bot can seek out it's charger; I read PIN 13 to see if the bot has managed to actually hook up to the charger. The circuit works great, but I am hoping to a) eliminate the schmitt inverter to save some board real estate, and b) to figure out some ground rules about when it is truly necessary to protect a Stamp pin with a limiting resister. Does that make sense?

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When the going gets weird, the weird turn pro. -- HST

DaveG

Zoot,

Regarding the Schmitt trigger, I think you can easily combine the comparator and the Schmitt trigger
with the addition of a single resistor. Take a look at the National LM3900 data sheet. It shows a Schmitt
trigger diagram.

However, a software Schmitt trigger will do the same job.

Regarding protecting a Stamp pin, I would suggest using a series resistor of at least 220 ohms whenever
you are connecting a Stamp pin to the output of another device, such as a comparator or logic gate. This will
prevent an accidental overload on the Stamp pin in case you inadvertently make Pin 14 an output.

DaveG

Zoot

Thank you, thank you.

DaveG said...
I would suggest using a series resistor of at least 220 ohms whenever
you are connecting a Stamp pin to the output of another device, such as a comparator or logic gate. This will
prevent an accidental overload on the Stamp pin in case you inadvertently make Pin 14 an output.

This answers such a basic newbie question for me, thanks again. I have been wondering why on some circuits I see there is protection between the Stamp and a TTL circuit and sometimes there isn't....

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When the going gets weird, the weird turn pro. -- HST