Simple connections to the AXE117 project board

BruceT

New Member
Hi, I have never done this before, so please bear with me!!!

1) I need to turn an external circuit (about 4 volts) on and off. Is there a circuit diagram that I an refer to for this that shows me a layout?

2) I have a number of inputs - pins that I need to set high or low using internal voltage. I expect that this would be quite simple, connecting a switch between +5v and the pin via a 10k resistor? I just can't find an exact diagram to show me this!! When they turn on, I want them to go high.

Maybe next post I will have something more challenging!!! I doubt it though!

cheers, Bruce
 

AllyCat

Senior Member
Hi,

Sorry we (or at least I) need to ask some more questions:

As it's an AXE117 with a battery connector cable, do we assume that the Power Supply is three AA cells in a battery holder, i.e. (nominally) 4.5 volts? How much current (mA) does your "external circuit" require? If you don't know, what is the "circuit" that you want to drive? Note that the outputs from the Darlington chip on the AXE117 only pull "Down" (towards earth), they do not pull "Up" to any supply rail. The direct PICaxe outputs can pull Up or Down, but don't assume that any more than 10 mA is available.

Are your "inputs" to the PICaxe, or to the external circuit? The PICaxe inputs appear to have pull-down resistors on the PCB, so yes, for those you can use a switch to the supply rail with preferably a series resistor of 1k - 4k7. For circuits not on a pre-designed Project Board, personally I prefer switches to pull down, with resistors to the supply rail (possibly even the Weak Pullups available inside the PICaxe) to pull the input(s) normally-high.

Cheers, Alan.
 

BruceT

New Member
Thanks Buzby, I did read that but will look again.

Alan, I am firing a camera and need to close two separate circuits. The first is the "1/2 press" or focus circuit and the second is the one that actually fires the camera shutter. It's important that I separate the two functions and have already worked that part out. I now need to close those two circuits. They are both about 3.5 volts.
For power to the project I was thinking about using 4 rechargeable AA's - that should be about 4.8 volts (1.2 from each, and I won't be able to use any other type of AA which is fine).

cheers, Bruce
 

techElder

Well-known member
I won't be able to use any other type of AA which is fine
You could also build a "dummy" AA battery to put into one of the cell slots. This would allow you to use three alkaline AA cells also. (The inevitable always happens! :D ) Be sure you have a good common connection between the camera and the PICAXE circuit.

They are both about 3.5 volts.
We're not sure whether you are pulling that "3.5 volts" to common or to "3.5 volts."
 

BruceT

New Member
Ok, I am not sure what you mean (even though it's a very basic question!!!) ... If I have the camera circuit earthed to the picaxe ground (0 V), then I would be pulling the 3.5v to common?

Yes, good idea about the dummy battery!!! Ta.
 

lbenson

Senior Member
I have a number of inputs - pins that I need to set high or low using internal voltage. I expect that this would be quite simple, connecting a switch between +5v and the pin via a 10k resistor? I just can't find an exact diagram to show me this!! When they turn on, I want them to go high.
Couple of questions/thoughts about this. First re "inputs - pins that I need to set high or low using internal voltage": are you saying that based on inputs you need to set outputs high or low?

And "connecting a switch between +5v and the pin via a 10k resistor". If you do that, you would also need a resistor to 0V (say 100K) to pull down the pin's voltage to 0 when the switch is not connected to 5V. Otherwise, the pin would "float", perhaps picking up ambient mains AC fluctuations or other, and be indeterminent as to whether high or low.

A slightly easier option for an input is to use the PULLUP command to make the input pin normally high, and have your switch go to 0V (perhaps through 10K to prevent an accidental setting of the pin HIGH and then with the switch shorting the pin to 0V). This inverts the logic (low means active, high means inactive), but that can be taken care of in software.
 

techElder

Well-known member
If I have the camera circuit earthed to the picaxe ground (0 V), then I would be pulling the 3.5v to common?
Someone else may know offhand, but only you know if that camera input is active low or active high. I would guess that it is active low — you pull it down to common. Maybe not.
 

oracacle

Senior Member
you could use a transistor pull the signals low (at least if it a Nikon or canon camera). However my preference on camera projects in the past has been to use opto isolators. One of the main reasons for this is they do what it says on the tin, isolates. if there is problem with my circuit then I don't take any part of the expensive camera out.

two of my camera projects:

this may also be helpful: http://axotron.se/blog/time-lapse-shutter-release-based-on-arduino/
the main consensus seems to be use an opto isolator for camera interface. I hacked and remote shutter release cable for use with my projects, just cheap no name ones from china.
 

hippy

Technical Support
Staff member
I am always cautious in suggesting how to interface to an unknown circuit because it may not work exactly how it appears to be externally.

I would guess that as the two inputs to the camera are floating at around 3V5 they are pulled high to the battery rail and will activate when pulled low. It may be worth trying that by pulling the pins low, to camera 0V ( -Ve battery terminal ? ) with a 1K, maybe reducing that if it doesn't work.

If it's a pull-to-0V to activate that can be implemented with a direct PICAXE connection to an opto isolated buffer depending on how safe one wants to be.

For a keyring photo frame which has the same pull-to-0V activation, I simply connected PICAXE 0V to photo frame 0V, connected its buttons to PICAXE I/O pins which I set as "INPUT x.y" when not activated, "LOW x.y" to activate. Works a treat.

For extra safety a resistor in-line would be advised, even a diode with its pointy-end cathode facing the PICAXE, or both.

That's a $1/£1 item and not the end of the world if it did burst into flames. I might be a bit more cautious with someone else's expensive camera or similar, would recommend a transistor / FET / opto / relay interface.
 

BruceT

New Member
Thanks everyone! Too many comments to respond to each one! The project by Oracacle - high speed photography - is pretty much what I am doing, so I might check that out. An opto isolator may well be the go!

cheers, Bruce
 

BruceT

New Member
I just went back to Manual 3 which discusses interfacing circuits and I see on page 6 that there is a circuit diagram showing a transistor interface. So that's all very good, but my board is the AXE117 project board and it has a ULN2003A Darlington chip on it. Part of the problem I have is that I am not quite sure what specs that Dalington has and how it might relate to that diagram on page 6. If it consists of a set of these transistor interfaces (and back EMF supression) then what sort of current / voltage can it handle? Can I simply use it to fire a camera shutter?
Also, there is an existing set of interfaces for inputs (LHS), but their layout is different to what everyone is telling me. If I connect a switch between those terminals on the far LHS, I think when I press the switch the pin goes high (?).
I have attached the AXE117 info for reference.
 

Attachments

BruceT

New Member
I should have looked at the next page!!! It deals with the Darlington. That might be answering most of my questions. I need to work out how much current flows in that camera shutter circuit.
 

AllyCat

Senior Member
Hi,
half an amp at 50 volt - so good for most.
I was surprised to see that the AXE117 appears to connect the ULN 2003A's V+ pin to the PICaxe supply rail, so effectively the output is limited to 5 volts, but not an issue for this project / thread.

Because Darlingtons have two transistors (' base-emitter junctions) in series, the lowest pull-down voltage is normally about half a volt, but again not likely to be a problem for this project. At very low currents (a fraction of a milliAmp) the pull-down voltage may be lower because the input transistor can operate independently, in series with the pull-down resistor in its emitter.

Finally, don't forget that an opto-isolator DOES need an input series resistor (unlike the ULNs). A direct PICaxe output might limit the current to a "safe" level (not recommended, however) but the Darlington will not !

Cheers, Alan.
 
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inglewoodpete

Senior Member
For power to the project I was thinking about using 4 rechargeable AA's - that should be about 4.8 volts (1.2 from each, and I won't be able to use any other type of AA which is fine).

cheers, Bruce
Note that NiCd and NiMH cells are just over 1.4v when fully charged; typically 1.3v for most of there working life and around 1.2 volts when "flat".

4 x 1.4v = 5.6v, which OK for a PIC: just don't bank on the supply voltage being a maximum of 4.8v.
 
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