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Need Circuit help for Airsoft Project

  1. Jul 4, 2005 #1
    I am posting here because someone I know needs help designing a circuit for an airsoft project. I have taken some high school electronics classes but I cant seem to wrap my brain around this one. Basically Airsoft involves replica models of weapons that fire plastic BBs in a game similar to paintball. These replicas have 3 fire modes, Safe- The gun cannot fire, Semi Auto- The gun is limited to firing one BB per trigger pull based on a mechanical restriction and Full Auto in which it fires as long as the trigger is held down. It does this by having the trigger complete a circuit which turns a small electric motor which operates the firing mechanism.

    The problem is that he wants to add a 4th firing mode which will cause the gun to fire a burst of 3 BB's per one trigger pull. I can see that a circuit will be needed to cause an output to go HIGH for about 3 seconds and then go low, no matter how long the trigger is held, until it is released and repulled. The other problem is figuring out what combination of resistors and capacitors is needed to attain the desired length for the HIGH signal. Any help regarding schematics and parts would be most appreciated.
  2. jcsd
  3. Jul 5, 2005 #2


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    A 555 timer would be a simple and inexpensive way to build any timer that doesn't need to go for too long a period of time (seconds ok, minutes get sketchy). Note that it can either deliver continous pulses or a single pulse.

    You can search and find more info, here is a very informative page with example circuits.


    EDIT: This IC wouldn't deliver enough current to drive a motor (its about enough to light an LED) so you'll need to use a transistor or a relay to convert its output to one suitable to drive a motor.
    Last edited: Jul 5, 2005
  4. Jul 5, 2005 #3
    Ok I think I've got it. Does this schematic look right for a delay of about 3 seconds? Also are 2ohm resistors and 2uF Capacitors too small a value to find?

  5. Jul 5, 2005 #4
    I hate to be the fly in the ointment here but I don't think a 555 will even drive a relay by itself. You probably need a darlington between the 555 output and the relay. Try getting a ULN2003A if you can. Also you have the relay going right to the 555 timer output---this is bad for most IC's because the inductive kick will destroy the 555 timer. If you forgo the darlington you at a minimim need a flyback diode to prevent hours of headache and destroyed parts. http://www.acroname.com/robotics/info/articles/drivers/drivers.html

    For your resistor/capacitor values you might want to use this because I don't think you values are right:

    You can invert the output if you need to and use that to drive the darlington.

    Good luck.
  6. Jul 5, 2005 #5


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    Good catch faust9 - I was thinking of one of those solid state relays. IIRC the 555 is good for like 20mA or so, but some are so fragile I've killed them on power-up at just under their max rated voltage so the vendor/batch could come into play too.

    Blackmesa 2 ohm * .000002F = 4 microseconds = 1/250,000th of a second! You may want to use the calculator faust9 linked to above. :smile:

    Yes, they're easy parts to find. A reasonably priced place with no minimum order is http://www.mouser.com and they have something like the ULN2003A referenced above for $0.42 so that's pretty reasonable.

    If you're lucky you might find a RadioShack that is stocked with parts and has a helpful staff member, but this is highly unlikely since their strategy is no longer serving the tinkering market and instead selling high profit items like batteries and cell phones. A couple years ago I had to drive to 2 stores just to buy the last package of 2N2222 style transistors because I was trying to finish something on a Sunday, very frustrating.
  7. Jul 5, 2005 #6
    I added a darlington and a protection diode and fixed the capacitor and resistor values. I know the part number on the darlington is wrong, circuit maker didnt have the number you suggested. My only question is how to figure out the value of R2 from my schematic. This site http://www.kpsec.freeuk.com/trancirc.htm has a formula that leads to it but I dont have all the values, such as the resistance. Is there a way to guess or should I measure them (I dont have the stuff infront of me)


    Yea RadioShack isnt all that great, my teacher used to say "You've got questions, and we've got even more"
  8. Jul 5, 2005 #7
    The PN on the Darlington is correct. They can be had from mouser(mentioned above) or Jameco. If you use the Darlington then there is no real need for an external diode because the Darlington itself includes one internally. Go to the site I linked to. On that site is a box that lets you ennter your desired time delay and then spits out a matrix of resistor capacitor values.

    Good luck.

    [edit] To figure out the value of R2 you need to do a voltage loop. The darlington datasheet should tell you the saturation current and voltage drop across the transistor. Make sure you 555 timer can source the saturation current(that is why I'd us the chip I recommended because it is designed to operate from TTL logic levels). Use the saturation current and the voltage drop across the darlington to determine the resistor value. You'll have to get your hands on a low voltage relay or source the darlington from a higher voltage source. Also, your darlington arrangement is incorrect. Draw a loop throught the the transistor circuit (emitter to collector) and you'll see you have no power source for you relay. The easy solution would be to connect the relay to the emitter and the PS to the collector.

    Back to resistor determination: Lets say the Darlington drops 2.5V of a 5V supply base to emitter at 20mA. That means the resistor must drop the remaining 2.5V at the same 20mA. Use Ohm's law to determine the resistance. Disregard the relay because the C-E current will dominate and the R of the driver coil should be relatively low. That's what I would do at least.
    Last edited: Jul 5, 2005
  9. Jul 5, 2005 #8


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    Couple more quick things:

    You have the motor on the NC contact of the relay, likely want them on the NO contact.

    Also you might want to swap the 5.8k resistor for a 1k and a 5k linear potentiometer so you can fine tune the timing over a much larger range of time in case 3 seconds isn't exactly correct.
  10. Jul 5, 2005 #9
    Im still confused on a couple things. My memory of certain things is failing me :) I cant seem to find the saturation current or voltage drop form the sheets. Do you have any recomendations for either a specific low voltage relay or a specification I should ook for? Or if I use a higher voltage on the darlington, what should I use to make it work but not destroy it.
  11. Jul 5, 2005 #10

    Look for the spec labeled V_BE(sat) which is 0.5mA. At the saturation current the voltage V_BE is 2.0 volts. Your voltage drop across the CE junction will be about 1.5V@500mA so account for that also. If you use a 5v relay it will need 5v to operate correctly. Go to Digi-key, Jameco, or Mouser for parts if you don't have a lockal electronics shop.

    We're talking about a small motor are we not? You can loose the relay all togeter and drive the motor via a TIP120. You could even use a mosfet as a switch however, mosfets are pretty sensative.
  12. Jul 6, 2005 #11
    So for the resistor would it be 3.5v/.5A? Also should the darlington be the ULN2003A or the one you just posted the data sheet from. I dont know the motor draw but its probably around 9.6v with maybe a 1700mAh battery. I can find that out for sure though later. Would a TIP120 be able to handle something like that. Also will the output only remain high for those 3 seconds as long as the trigger is held high or can the trigger return to low yet the output will be high?
  13. Jul 6, 2005 #12
    Not .5A, .5mA. The pdf above was for the darlington you have in the schematic. I'd use the 2003 personally. Motors don't draw voltage they draw current and the 1700mAh is how long the battery pack will last i.e. if your motor draws 1A then the battery pack would last 1.7 hours(not really though). A TIP120 can easily handle a small motor. Add a heat sink and use a 390ohm 1/4W base resistor and it should work.

    I'll have to read a 555 timer data sheet before I answer your last question. I have not used a 555 in quite some time because I find PIC12F675's much more functional for only a slight increase in price. I 'think' the copnfiguration you have is non-retriggerable meaning pull the trigger and the system will do its thing for 3 seconds no matter what you do after the initial trigger pull.
  14. Jul 6, 2005 #13
    Im going to go with the darlington you suggested, the only reason I had a different one in the schematic is becuase Ciruit maker did not have that model. Judging from the data sheet here: http://www.st.com/stonline/books/pdf/docs/5279.pdf It looks like it is more than one darlington in one IC. Would that mean that IN 1 is the base, OUT 1 is the collector and Pin 9 is the emitter? What is the TIP120 referred to as? I cant seem to find it in Circuit Maker. PIC12F675 A PIC is one thats programable right? Is that really necessary for a project like this and if so is it hard to program? Thanks so much for your help so far.
  15. Jul 6, 2005 #14
    I have another question. Is it possible at all to do anything like this using a single capacitor. I know that in series it will store energy and then stop and that in parallel it will discharge the stored energy as needed but is it possible to have it store the energy and then discharge it even if the draw does not change?
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