# low current relay or transistor?

#### blittle

Joined Oct 6, 2006
6
Sorry in advance for this very basic question.

Ok, So you've seen those teeny tiny remote control cars that you connect to the transmitter to charge?

I removed the circuit board from the car and clipped the motor leads which I want to use to trigger an electric ignitor for a model rocket engine.
(the rocket engine is mounted on a full size RC car, but that's beside the point)

I measure the full 1.5vdc at the motor leads while the transmitter is operating, but have no idea what current is available; besides not enough to 'burn' the ignitor. I tried using the smallest relay I could get at Radio Shack to provide full battery power to the ignitor but no go. The motor output won't operate the relay. I think I should use a transistor? What type / rating etc should I use and how would you wire the circuit?

#### Papabravo

Joined Feb 24, 2006
16,829
Don't the people who make the model rocket have such a device already to go. Why are you messing around with ad hoc Rube Goldberg contraptions?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

The fuze probably has a spec on it that requires so much current at so many volts to ignite. If 1.5 volts won't do it, then your only way to go is to supply the nevessary current from a higher voltage source.

#### blittle

Joined Oct 6, 2006
6
No, Estes does not sell an RC ignitor, presumably because the right 49Mhz interference would trigger your ignitor. i.e. Not safe.

Yes, the remote (wired) ignitor trigger uses 6vdc, so I need at least that to burn it. The plan is to use the 1.5v output from the receiver board to trip a low power relay or transistor switch along with a 9V battery to provide power to the ignitor.

Using a relay you can simply wire the output leads to the 2 relay input terminals, with the 9v battery and load on the switched terminals; But transistors only have 3 leads so I don't know how you would wire this. Or what type to use.

I don't mean to dumn-down your forumn, I just need some basic help and could't find a better electronics board to post on.

BLittle

#### blittle

Joined Oct 6, 2006
6
Would this work?

#### mrmeval

Joined Jun 30, 2006
833
I would add a manual switch at the base drive that killed the circuit and run that a bit away. That way when another remote tries to signal a car your rocket doesn't ignite with you in the way.

There are solutions to RF launching, the better ones would use a 32 bit numeric identifier and the best would use a 64 bit keyed identification scheme. Not cheap by any means.

#### blittle

Joined Oct 6, 2006
6
Thats doesn't really help.

#### thingmaker3

Joined May 16, 2005
5,084
I humbly suggest BLittle's solution - but placing the ignitor between the collector and the positive battery lead. Might also want to add a current limiting resistor at the transistor's base.

No, Estes does not sell an RC ignitor, presumably because the right 49Mhz interference would trigger your ignitor. i.e. Not safe.
BLittle
How will you address this issue in your home-built unit? How will you keep the rocket from launching while still in your hand if the kid a block over is playing with their RC?

MRMeval has given a suggestion that is very helpful in this regard.

#### blittle

Joined Oct 6, 2006
6
How will you address this issue in your home-built unit? How will you keep the rocket from launching while still in your hand if the kid a block over is playing with their RC?
The rocket engine is mounted to the back of an RC car.
The clips to the rocket ignitor will only be hooked up when I'm ready to GO.

#### Chris Wright

Joined Jul 26, 2006
62
Here is one posable circuit:

This circuit has a Safety switch that keeps the R/C firing circuit isolated until the last minute. In the "Safe" position, this switch will allow you to hook up the igniter leads, power up the circuit and check the continuity.

To see how this circuit works, in a separate window, go here for a Java Circuit Simulator Applet (You can run it online or down load It):

Select "Blank Circuit" from the right hand "Circuits" menu
Select "import" from the Applet "file" menu in the middle of the menu bar.
Copy and Paste the netlist below into the import window and click "import".

Click on the Power switch and see how the continuity works. Mouse over the igniter (or any other part) and it will show you the voltage and current present. If you right click on a part, it will allow you to edit the value or delete it. If you right click on a blank area, a popup menu will appear with a list of parts and actions.

Click on both poles of the "Safety" switch (they aren't connected by the program) to arm it, and then click the R/C switch to see the voltage and current through the igniter.

You can use this Applet to fine tune the values of the resistors for the transistors and for the LED's, but it isn't too critical. RadioShack sells a package of 15 small signal NPN transistors and a pack of 20 LED's either one for $2.50 that should work fine. Revised Java Applet NetList:$ 1 5.0E-6 10.812258501325767 50 5.0 50
t 352 224 400 224 0 1 0.0 0.0
r 352 224 304 224 0 6800.0
162 320 144 400 144 0 1.0 0.0 0.0
162 320 96 400 96 0 1.0 0.0 0.0
r 320 128 256 128 0 330.0
w 400 144 400 208 0
w 304 224 288 224 0
w 288 224 288 176 0
w 272 176 272 224 0
w 272 224 192 224 0
w 400 96 432 96 0
w 432 320 400 320 0
w 192 224 192 112 0
w 192 112 256 112 0
x 326 172 342 172 0 14 Continuity
x 332 74 348 74 0 16 ARMED
R 192 304 144 304 0 0 40.0 9.0 0.0
t 240 304 288 304 0 1 0.0 0.0
w 288 224 288 240 0
r 240 368 240 304 0 100.0
v 320 432 240 432 0 0 40.0 1.5 0.0
r 320 320 400 320 0 10.0
x 129 275 145 275 0 16 Power
x 149 434 165 434 0 16 R/C 1.5 V
x 254 168 270 168 0 16 Igniter
x 347 125 363 125 0 12 LED's
s 240 432 240 368 0 true false
s 288 240 288 288 0 true false
w 400 240 400 320 0
w 288 320 320 320 0
x 312 275 328 275 0 14 Safe
x 233 275 249 275 0 14 Armed
s 192 304 192 224 0 true false
s 432 224 432 288 0 true false
w 432 96 432 224 0
w 432 288 432 320 0
x 463 273 479 273 0 12 DPST Safty switch
x 493 254 509 254 0 12 - 2nd Pole of
x 323 254 339 254 0 12 DPST
g 400 320 400 352 0
r 256 96 320 96 0 330.0
w 256 96 256 112 0
w 256 112 256 128 0
w 320 128 320 144 0
r 192 112 128 112 0 330.0
162 128 112 128 176 0 1.0 0.0 0.0
x 104 203 120 203 0 14 Power On
g 128 176 144 176 0
w 272 176 288 176 0
w 320 432 320 320 0
x 399 228 415 228 0 12 Q1
x 156 136 172 136 0 12 R1
x 303 87 319 87 0 12 R2
x 290 307 306 307 0 12 Q2
x 303 120 319 120 0 12 R3
x 318 203 334 203 0 12 R4
x 211 344 227 344 0 12 R5
x 354 344 370 344 0 12 R6
x 170 241 186 241 0 12 S2
x 296 253 312 253 0 12 S1a
x 465 255 481 255 0 12 S1b
o 7 64 0 3 7.62939453125E-5 9.765625E-5 0
o 17 64 6 3 7.62939453125E-5 9.765625E-5 1

#### mrmeval

Joined Jun 30, 2006
833

#### thingmaker3

Joined May 16, 2005
5,084
I repeat my question - this time worded a little differently for clarification:
How will you prevent random transmission from similar toys from igniting the thing while you hold it? How will you be certain to avoid burning yourself as a result of 49MHz interference?

And a new line of questions also:
Have you read Estes' commentary on why horizontal projectiles are a bad idea? How will you keep the thing from bouncing off a bump in your "race track" and going into someone's face?

#### blittle

Joined Oct 6, 2006
6
Chris,
Wow. Thank you. Maybe version 2?

thingmaker3,
This project is simply the result of having a bunch of RC junk around. The rocket engines sizes and types are clearly documented for their thrust and burn time. So you can choose something that won't be _completely_ out of control. The weight ratio of the RC car compared to a rocket is hugely different. It will not go airborn. To answer your question about how I will prevent interference from activating the circuit while I hold it - the ignitor will only be hooked up when ready to fire. So safety is more a matter of procedure than design.
Version 2 could be Chris' design, or maybe a second mini transmitter for an 'arm' relay? In any case, there is a large smooth paved area just around the corner and no worries about hitting bystanders.
p.s. Estes does sell horizontal projectiles. http://www.estesrockets.com/Funny_Car_with_Digit1261.html

Blittle

#### thingmaker3

Joined May 16, 2005
5,084
Its good to see you thinking about precautions and doing the math. I did't know plugged engines were now available - been about thirty years since I bought from Estes.

Still, I strongly advise some kind of failsafe to keep the thing from igniting in your hand. A simple SPST switch in series with the ignitor would work - hook up the ingitor with the switch open & close the switch when flesh is clear.