Hi guys, I'm hoping someone on hear can help me with a circuit I'm trying to build - I've said I'll have a crack at modifying my mates airsoft gun to give it a single shot mode controlled electrically.
My first version has the motor power flowing through a Mosfet which is activated for a limited time by a capacitor discharging through a resistor when the trigger is pulled. When the trigger is released the capacitor recharges ready for the next shot, and the length of time the motor runs is determined by the relationship between the capacitor and the resistor.
The big issue with this set up is that as the charge in the battery decreases so does the length of time the Mosfet activates for, and also the motor runs slower, with the end result that the motor doesn't complete a full shot. I can tweak the capacitor and resistor to compensate for this, but then it tends to fire more than once on a full charge!
Is there any way around this? I'm wondering if increasing the capacitance (and reducing the resistance suitably) would make it less sensitive to voltage changes? Or is there a better way that I haven't thought of?

Cheers guys

[Edited on 27/12/14 by Badger_McLetcher]

I would suggest making the timing part of the circuit voltage independent by having a voltage regulator. .. 7805 or something. Then as long as the battery was higher than Vreg+0.7v then the timer circuit would always give the same times.
Hth

555 timer these are constant current devices regardless of supply voltage.

I would of thought that a more timing specific chip like the 555 would work better for this. There are others but I'm rusty on this non programmable side of circuit building.

The 555 will have a chandler timed output method which matt allow for a three spot burst of the gun allows it

http://555-timer-circuits.uk/operation/one-shot.html

Love that web site, now that is a contribution!

Awesome, cheers guys! Looks like I've got a bit more research to do I've only got a limited amount of space to play with so it may be fun trying to fit the timer in

[Edited on 27/12/14 by Badger_McLetcher]

Surface mount?

space,... pah

This is my radio microphone,
fist_mic standard on the outside, but contains amongst other things:

Parrot - audio recording and playback device
Vox - voice activated transmit switching
auto repeat for the parrot - with variable interval timing (surface mount 555)
hard switch audio lines - to isolate the live mic when the parrot is "talking"
hard switch transmit line - to ensure fully switched transmitter
some tape, some blutack, and a press to talk switch......

the device in the foreground in focus is a 700mA opto-isolated relay for the transmit line switching



Oh and it was first built in 1995 and has been modified a few times since,...

I am sure that there is a more tidy solution, but this one is very organic

[Edited on 27/12/14 by HowardB]

I believe Vero board is your solution.

Pretty sure single 555 timer is available in 8 pin DIL package, couple of resistoras and capicitors would fit in area of 5p coin.

You can wire up without using a circuit board to save space then encase in a cube of potting compound

http://www.electronics-tutorials.ws/waveforms/monostable.html


Here is a useful page for alternatives to the overly relied upon 555..

555 great IC but eats power compared to modern equivalents.