Momentary-activated timer with no idle power consumption

[KMoffett]

....Will this just briefly pulse the relay then only reset on power loss?

Yes. When the power is applied the capacitor looks like a dead short and the relay is energized. As the capacitor charges the relay loses voltage and drops out. When the capacitor is fully charged, it draws no more current. When the power goes away the capacitor discharges through the diode and resistor. Pick a relay with the highest coil resistance, so you can use the minimum size capacitor...therefore the least total power consumption.

Ken

 

[w1zard]

Hi Ken
Thanks for the explanation - I understand, and it all makes sense now!

Regarding your circuit, I have found a small but expensive quantity of BS250, but it's not a generally stocked item in a lot of the suppliers I use. What is the important figure on the data sheet to look for? Would something like a ZVN3306A be compatible? Why is it that you've not used a 2N7000 here - is it the ID value that is different? I'm struggling to isolate the difference between the transistors.

Thanks

 

[KMoffett]

The BS250 is a P-channel MOSFET. The ZVN3306A and 2N7000 are N-channel MOSFETs. This is analogous to the difference between NPN and PNP transistors. They aren't interchangeable. You can probably use almost any small P-channel MOSFET with a low Rds. Since I don't know where you are, I can't reference a source for an appropriate replacement.

Ken

 

[w1zard]

Sorry - you're right, I'm looking in a paper catalogue and it doesn't make it clear which are the N-channel and which are the P-channel. I'm in the UK, and use Rapid for large orders, but mostly get small sets of components from Ebay, as their postage is more reasonable. I also use bitsbox.co.uk a fair bit - they have a ZVP2106A - would this okay?

I've not used MOSFETs to a great extent - are these similar in use to BJTs but with a higher current capability? If so, what part of your circuit dictated the use of these instead of normal BJTs?

Thank you

 

[KMoffett]

The ZVP2106A should work.
MOSFETs do not require drive current to turn them on, except to charge the small gate-source capacitance at turn-on. The P-channel could be replaced with a PNP transistor and the N-channel with a NPN transistor. In fact that's how I originally designed the circuit. But, since I was looking for the lowest possible battery drain in that project, I went with MOSFETs. You would need a current limiting base resistor between the uC output and the NPN's base, a pull-down resistor from the NPN's base to common, and the resistor values would have to be changed.

Ken

 

[w1zard]

Ah - I see - so they are basically a more energy efficient transistor because they only use current on switch-on.

The in-use current is not that important in this application - it's the idle current that's important as it might sit idle for a long time. The battery will cope with a minute or two of drain each day without a problem. To keep it simple, I'll stick with the MOSFETs, as I've ordered the N-Channel ones already, but it's useful to know for the future that I can interchange them with a few amendments.

I've managed to find a circuit lying about with a BS250 transistor in it, so I'll give the circuit a go once the other parts arrive.

I'm currently printing the PicAxe manual in all its 107-page glory, so I have a bit of homework to do now!

 

[w1zard]

Hi Ken
After much reading, playing, learning, cursing, etc, I have your PicAxe momentary switch circuit prototyped on breadboard and it works great

I reduced the pause time for the switch debouncing, which improved the responsiveness of the switch, without affecting the performance. At this point I've only got it driving an LED rather than a relay, to simplify the prototype, but don't anticipate any problems adding this later.

As I don't have PCB manufacturing facilities (maybe one day!), I'm going to transfer the circuit to veroboard. As it's pretty complex on breadboard (it's a mess), I'd like to design the veroboard layout in CAD first.

What software did you use to draw the schematic? Is it possible to have the original file for it? I'm currently deciding which bit of software to learn, I'm playing with LTSpice for simulation, but it seems Eagle is pretty popular for designing layouts, and it can be tweaked to do veroboard from what I have read. Would you recommend it? Is there another veroboard prototyping package that can import/create schematics and auto-route them onto a veroboard?

Any hints would be appreciated, as I'm probably going to have a bit of a learning curve, and would rather invest my time in the package that I'll continue to use (or buy).

Thanks

 

[KMoffett]

Happy to hear it's going well.

I used ExpressPCB to do the schematics.
>FREE!< ExpressPCB - Free PCB layout software - Low cost circuit boards - Top quality PCB manufacturing
I also use it to transfer designs from the schematic/breadboard, on one-off projects, to solder-pad perfboard. If I decide to later go with a PCB, I can just send it to their board house. For the perfboard layout I had to add components that have 0.1" spaced footprints to their "custom library", to match the perfboard grid. Easy to do.
ExpressPCB does link the schematic to the PCB for net list verification, but doesn't do auto-routing. Eagle does auto-routing and allows you to send you design to any board house. I don't know about it's ability to rout to veroboard footprints. If you chose to go with ExpressPCB, I can send you the components in my custom library and the schematic file. However, the PCB file was for a circuit that included other things that would just confuse things for you.

I would be happy to help as you proceed.

Ken

 

[w1zard]

Looks my like my reply to you disappeared into the ether...I'll try again

Thank you for the info - I'll take a look at ExpressPCB and Eagle and decide which one I'll learn in more detail, and let you know should I need the original files. Autorouting would be a very useful feature on Veroboard because I'm a bit space-constrained, and a computer will always make a more compact job of the routing than I would!