I m working on a 1/72 Gato submarine and I need help with a piece of electronics. What I am doing is building a torpedo that will run for 10 seconds and shut off. Currently there are a lot of torpedoes made to run off of compressed gas, I do not want to use gas. I am trying to build it using a reed switch that when slid into the torpedo tube a magnet will close the reed switch. Then to launch, the magnet is pulled away and the torpedo begins its run for 10 seconds. I am going to have to use small batteries ( 3v, maybe watch batteries???) the motor will be from a toy R/C helicopter.

I was told that it is a simple circuit using a 555 chip....... I am just electronic vacuum.

Can anyone help me with what I need to do to accomplish this?
Thank you!
Peace,
Tom

 

This is close to what you want. It's called a one-shot circuit; the time is controlled by R1 and C1. The values shown will produce a 10 second time. In the circuit shown, the one-shot lights an LED for 10 seconds, but it could control your motor circuit. As shown, the circuit includes some components that are recommended, but not always used. Power is show as 12 V, but it will run on much less. No doubt there are others here who will suggest changes.

 

WOW, tracecom you are amazing! Thank you! If I understand, the lower voltage will use the same components? So, I can use this as a shopping list?

 

WOW, tracecom you are amazing! Thank you! If I understand, the lower voltage will use the same components? So, I can use this as a shopping list?

Not really amazing. It just happens that I have been experimenting with NE555's lately and have that circuit prototyped on my desk.

The only component that would change is R3, but you probably won't be using that anyway. Before you start shopping, you should post more of the details about the motor you are going to use especially the voltage and current it requires. It may need a transistor driver depending on the current it requires. In addition, you should know that the timer starts on a negative going signal on the trigger lead. In my circuit, that is accomplished by closing the normally open switch, but you may need something different depending on how your reed switch is going to be wired. Post a drawing of how you plan to handle the switching and the motor drive and that will let others here have a chance to comment on your plans and the circuit that I posted.

There may well be better ideas.

ETA: The NE555 needs at least 4.5 V to operate.

 

I will try to find out the specs on the motor and post it here. Is there any difference if I try to build this with smd? and what resistor would I change if 10 seconds is too long or not long enough?
Thank you again, this is like a Christmas present to me, I am really excited that this might work!

 

I will try to find out the specs on the motor and post it here. Is there any difference if I try to build this with smd? and what resistor would I change if 10 seconds is too long or not long enough?
Thank you again, this is like a Christmas present to me, I am really excited that this might work!

SMD would reduce the size, but it's more difficult to solder. In addition SMD components are normally used on a PCB, as opposed to a perfboard or breadboard.

The length of time that the output of the NE555 (pin 3) is high depends on the combination of R1 and C1 according to the following: time in (seconds) = 1.1 times R1 (in ohms) times C1 (in farads). As in the schematic posted, 10 seconds = 1.1 times 910000 ohms times .00001 farads. (Note that 10 μF can also be written as .00001 farads.) So, to make the time longer, make C1 and/or R1 larger, and to make the time shorter, make C1 and/or R1 smaller. (However, R1 tends to get hot if it's much smaller than 1k.)

It's good to be on the trail of a solution, but sometimes it's better to think things through rather than charge ahead. What is your experience at building electronic circuits? Do you have the minimum tools: a good soldering station and a DMM?

 

My experience in this is very limited. 20+ years ago I did mini kits and misc LED lighting. I do have a good soldering station and am trying to find my digital multimeter as we speak.
I never excelled in electronics - I tried converting my Radio Shack robotic arm to be controlled by my 386 processor PC by the serial port (it did not fully work) That was way back in the early 90's, but for the most part it was small LED lighting for the model kits.
Sorry about just rushing in...... I tend to do that.
Peace,
Tom

 

The LMC555 is a CMOS version and it will work with the calculated parts for the 10 seconds. I use that version on a number of projects that are powered by 3V lithium batteries. Pin 3 output will not drive much current at 3V so I use a 1.5K resistor to the base of a 2n3904 to boost the current. This version of the 555 will work down to 1.5V.

 

What are the dimensions of this torpedo? If it's really 1:72 scale, they will probably only have about a 1/4" inside diameter; you'd have to use AAA batteries, and a DIP package certainly won't fit in there.

 

It works out to be 7.4mm or .29" so yes about 1/4" I was looking at watch batteries as a possible source. I would probably have to create this ultimately using smd and on a circuit board. any thoughts or suggestions?

 

N cell 11.4mm X 29mm
AAA 10.25 X 44.4mm
pager vibrator motor 7.3 X 17mm

 

@SgtWookie, I like your name. I have a student who is graduating this year, he has been in my Network Admin program for 4 years and we call him Wookie. I will miss him!

@Bernard, It will not be easy, I could (looking up for lightning to strike) not go scale...... I hear the gasp, but the effect would be more important.

Thank you all so much for your input. I have not had much time to look up the motor specs. As soon as I do I will post.

 

O.K. here is one set of motors that might work:

Weight 3g | 0.1 oz
3.7 Volts
Motor Type: Coreless
Motor Length: 16mm | 1 5/8"
Motor Diameter: 7mm | 9/32"
Shaft Diameter: 1mm | 0.039"
Shaft Length: 5mm | 3/16"
These are sold as replacement for the Micro Mosquito Firefly Helicopter http://www.ebay.com/itm/Micro-Mosquito-Firefly-Helicopter-Main-Drive-Motor-Pair-/140564227588?pt=Radio_Control_Parts_Accessories&hash=item20ba47e604#ht_2214wt_1241

If you want to see my build of the sub it is here: http://forum.sub-driver.com/showthread.php?1652-Newbie-no-more-Building-the-Revell-1-72-Gato

 

How are your hands and eyes. You are going to have build this under a magnifying glass if the max dia is ¼". I have bought pager motors that would probably work.

I suspect you are going to have to free wire this without the benefit of a PCB, using mostly SMT or very small components such as 1/8W resistors (SMT is better). No PCB at all.

I need to reread the post to see how you plan on triggering this, that seems to me to be the primary challenge. Reed switches are pretty massive for this scale, so would seem to be out.

*************

OK, I notice you are indeed planning on using a read switch. I can simplify your schematic a little, which will help.

Have you drawn what you are planning on doing with all the other components? This is going to be critical.

 

Yes I can work under magnifier and with my hands. I am such a noob on this, draw it out???? no, not yet. To be honest I do not know what would work on this. So any help is appreciated.

 

I would build it except for the electronics to see how much room you have to spare.

Include the relay switch. If you could name the watch batteries you are going to use that would be helpful, they will be what determines the diameter of the torp. I would also like to see the reed switch you are thinking of using.

I am a big fan of drawing things to scale. This would seem to be a case where it would be a good idea.

 

The battery I am thinking of is SR66. It is 6.8mm x 2.6mm - the other one I was thinking of that is narrower is SR63, it is 5.8mm x 2.1mm. Probably would need 2 or 3 to work??
OOOhh I found a rechargeable called ACCU plus p10 power one

Type Designation
p 10 accu
Type No.
54610
Voltage (V)
1.2
Electrochemical system
Ni-MH
Typical capacity (mAh)
12
Diameter (mm)
5.8
Height (mm)
3.6
Weight (g)
0.3

O.k. reed switches -
I found this one:
http://www.cotorelay.com/html/reed_switch_ri-80smd.htm

Thank you for your help! This is so amazing.

 

Interesting, I did not know they could be so tiny, though I am not surprised. Let me think on it a little.

One last problem, nothing major. if you use batteries whose diameter is the same as the torpedo shell you have to figure out a way of getting the juice around the batteries. This suggests copper tape, which will still have a minimum thickness. When working with the really tiny you have to remember negligible thickness probably isn't.

There may be another way to do this that doesn't involve a timer. There are components called super caps that can be small. Charge one up, and you could get your set time out of the motor, though it will visibly run down. It would still involve electronics, just a different type, turn it on and it stays on until it is out of juice.

Funny thing, I help build microelectronics as an operator, I used to maintain the robots that did this. Here is a quick reference that gives you some idea of the scale of this process. Not the best reference, but I have not been able to find better over a long time, this stuff is pretty proprietary. What you want to do is a challenge even there.

http://www.semlab.com/imagesoffailuresinmicroelectronicspackaging.pdf

http://www.natelengr.com/Capabilities/hybrids.asp

 

Pager vib. motur will just fit in a 19/64 brass tube, 7.51mm OD, 6.75 ID, which would make length about 87.7mm. Motor-1.5V @90mA. What is current draw of Firefly motor? I like the J style reed- used hundreds of Coto reed relays early 1960's- they have been around for awhile.

 

Another approach is to use a SM 4060 which is 6.42mm wide & with 3 ext parts has internal clock. After 10 sec & 4096 th clock pulse, pin 3 goes high shutting off motor and disabling clock via D1[ not tested ] which suposed to keep pin 2 hi untill magnet is in place. When ckt is in standby it draws about 4 μA. R6 & R7 depend on motor drain. R4 168k may be padded up or down to give 10 sec run time.