PICAXE at 3V auto off

Dicky Mint

Senior Member
Hi

I've found this picaxe circuit for an auto-off design.

Clipboarder.2018.01.11.png

Could someone tell me if it is compatable with a 3V supply?

If not is there a similar circuit that would work at 3V?

Kind Regards

Rickz
 

StefanST

New Member
Yes, it wil work at 3V (and even at 2V).
If your circuit is powered by a battery, you should consider minimum battery voltage in the end of battery life.
Most PICAXEs work at minimum suplay 1.8V, 08M2 at 2.3V, 28/40X2 at 2.1V.
 

Dicky Mint

Senior Member
ok thanks that's great I must have done something else wrong as something is going pear shaped in my circuit!

I substituted BC847B and BC857Bs for the stated ones, i dont think that'd be a problem but could it be?

Regards

Rickz
 

techElder

Well-known member
Just saying that there is the possibility that you swapped the transistors. One is PNP; one is NPN.
 

Dicky Mint

Senior Member
yeah ok that's possible!

I'll re-check the identifying marks on the two transistors.

I'm going to have to break out the big guns again as my magnifying lamp and glasses are not enough with my eyesight!

short of employing my friend to use his MACRO lens I'm going to rely on a magnifying glass and glasses.

but probably should have double checked after I soldered them in!

Regards

Rickz
 

Dicky Mint

Senior Member
well I've double checked and they are the correct transistors in the correct places.

I'll have to systematically double check the circuit board untill I find the fault or faults!

Thanks for your help and advice

Sometimes its neccessary to go back to basics

Regards

Rickz
 

AllyCat

Senior Member
Hi,

There IS a subtle difference between having a supply above or below about 3.3 volts, if the "I/O" output is Low (not a "floating" input) in the quiescent state.

The 10k and 2.2k each side of the "touch" (push-button) switch form a potential divider when pushed with the PICaxe/Stamp output pin Low. With a 5 volt rail, the signal applied to the NPN base is about 900 mV which can turn the transistors On and power up the chip.

However, with a 3 volt rail, only 540 mV is applied to the base, which might not be enough to turn it on (and lower supply voltages would be more problematic). Not an issue provided that the Output pin is turned back to an input for the quiescent state, but there could be different functionality if the button is pushed (again) when the output is Low.

Cheers, Alan.
 

Dicky Mint

Senior Member
umm I think I follow you

would I therefore be justified in decreasing the value of the 10k resistor to say 2k2 also? thereby delivering 1.5 Volts to the base of the npn transistor? Or maybe increasing the origional 2k2 resistor to 10k?

1.5 volts should be ok on the base of the npn transistor?

I'm not sure I follow the argument concerning the 'on' button being depressed when the in/out pin is in the quiescient state!
Perhaps you could explain a little?

Anyway thanks for your help

Regards

Rickz
 

AllyCat

Senior Member
Hi,

Yes it would generally be better to "scale" the potential divider to match the reduction in supply voltage. I must admit that I was more considering the situation when the PICaxe is put to (indefinite) sleep rather than having its power supply totally removed (if it is*). But there should be a decoupling capacitor directly across the PIcaxe supply rails, so it's worth considering what might happen (or not happen) if the button is pressed shortly after the PNP switches off (but the reservoir capacitor has not yet fully discharged).

1.5 volts should be ok on the base of the npn transistor?
The base voltage never actually reaches 1.5 volts, because the base-emitter junction is a forward diode. If the potential divider tries to pull up higher than about 600 mV, then current flows into the base and is amplified by the transistor (which is its primary purpose, of course). So the base voltage will (should) never rise above about 700 mV.


* FWIW, I would rather dispute the "3 nA standby" claim. Is the "open circuit" resistance of the touch switch (and leakage current across any PCB tracks) really greater than 1G ohms (1,000,000,000 ohms) ? Also, the data sheet for the "2N3904/6 doesn't really give sufficient detail, but the collector-base leakage (Icex) is specified as "up to 50 nA" (personally I prefer Iceo in specifications). The specified leakage is at the maximum rated voltage, but at only 25 degrees C; we can only guess what it might be at 3 or 5 volts, but with elevated temperature. However, the leakage current can then be amplified by the NPN transistor (and then by the PNP), so the current fed into the PICaxe might be much higher.. To work well in "adverse" conditions one would normally add high-valued resistors (say 100k +) across most base-emitter junctions (to prevent any leakage currents being amplified by the transistors).

Cheers, Alan.
 

Dicky Mint

Senior Member
Cheers Alan for the in depth of electronic insight most of which is above my head!

I'll keep reading through it until it goes in... at least I'll try!

Steve G The simplified circuit is excellent

I thought it was pretty simple before.. although that was my big mistake, I learned!

I've got the origional setup designed into some PCBs, so for this project I'll stick to the origional circuit with the 2k2 resistor exchanged for a 10k.

Thanks again

Regards

Rickz
 
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