WOW!! That is certainly not what I had been imagining, and so once again we see that guesses will not always be useful at all. I was imagining an igniter for a natural gas heater, since many of them also use a hot element to ignite a gas jet to light the main burner.

 

The fit inside the 6ft long hull is tight because there are all kinds of servos, etc to control the sails and rudder.
So the MOSFET vs relay is a huge deal. Thank you all for getting me to that solution. Really appreciated.
CANNON SOUND...for those interested:
This model goes to on-water boat shows and kids love the cannons. The current ones are one-shot black powder and make only a slight pop sound. That's because, for safety, you can't have a high pressure barrel (needed to make a loud bang) at a public venue. This new e-sytem willl allow 30-40 shots per gun before requiring reload with "fog juice".
Speakers inside the hull not possible for four reasons - weight, power, hull is sealed (waterproof) so sound can't get out and lastly, sound travels very poorly over water. Just 20 yards out, even loud sound volume is lost.
NEW CHALLENGE...for those interested in helping with an electronics approach:
I want to set up an at-shoreline, high power sub-woofer and run a cannon boom sound file synced to the ships gun fire out on the water. The sound could be delayed...whatever... and the low freq subwoofer would give the impression that the sound came from the guns on the boat. Did a test of sound only and it works well...hard to tell outdoors where exactly the sound comes from.
The problem to be solved is how to activate the sound synced with activating the guns. Model is operated by an R/C transmitter, so I'm thinking an MP3player feeing the woofer. MP3 player would be activated by a servo on same channel as R/C switch that starts the gunfire sequence. Feedback and ideas appreciated.
Thanks!

 

The best faked boom would arrive very soon after the smoke puff, and so if you can get a switch closure as you send the command for a cannon fire to play the sound file, with just a small bit of silence at the start, that could have the effect without any added controls. Is the cannon fire signal from a button on the transmitter? Or what?

 

The best faked boom would arrive very soon after the smoke puff, and so if you can get a switch closure as you send the command for a cannon fire to play the sound file, with just a small bit of silence at the start, that could have the effect without any added controls. Is the cannon fire signal from a button on the transmitter? Or what?

Cannon signal requires three functions - all from a 3 position switch on one channel. But this can be easily re-programmed.
Three steps are: gun power on, port/starboard gun selection, and start firing sequence. Firing sequence is programmed om the MCU (as an Arduino sketch).

 

OK, it seems like the "start firing sequence" switch position would be the best source for triggering the cannon boom start signal. If that signal is available as an external connection from the controls transmitter that would be the way to go, but if not, then possibly another simple receiver to detect that command and provide the signal to the sound package would be a good choice. My experience with current complex RC systems is lacking, so I don't have much advice on that part.

 

OK, it seems like the "start firing sequence" switch position would be the best source for triggering the cannon boom start signal. If that signal is available as an external connection from the controls transmitter that would be the way to go, but if not, then possibly another simple receiver to detect that command and provide the signal to the sound package would be a good choice. My experience with current complex RC systems is lacking, so I don't have much advice on that part.

I think I'll simply add an R/C receiver and small servo rigged to actuate the "play" button on an old MP3 player. Will take a bit of playing with timing, but once set up, will be pretty robust.

 

I think I'll simply add an R/C receiver and small servo rigged to actuate the "play" button on an old MP3 player. Will take a bit of playing with timing, but once set up, will be pretty robust.

Y0uwill certainly learn about any variable delays in your system.. It will be interesting indeed.

 

An additional question please. What size resistor is needed in the above mosfet circuit?
"anything from 1 to 10k will do", but what wattage please. Is 1/2W big enough? Thank you.

 

An additional question please. What size resistor is needed in the above mosfet circuit?
"anything from 1 to 10k will do", but what wattage please. Is 1/2W big enough? Thank you.

Have a go at working out the wattage needed.
What is the max voltage across the resistor?
Then, P = (V*V)/R
Any resistor with a power rating higher will be ok.
Have you had a look at Ohm's Law?

 

Have a go at working out the wattage needed.
What is the max voltage across the resistor?
Then, P = (V*V)/R
Any resistor with a power rating higher will be ok.
Have you had a look at Ohm's Law?

Thank you. I see now that the answer is obvious (I think). I think I'm just overthinking the mosfet stuff...my first experience using them.
So... to check....if the signal voltage is 5V, then a 1K resistor would see .025W
Again, thanks.

 

Yet another rookie question please.
The shown simple circuit was set up on a protoboard and the voltages measured because the heater output was lower than when it was previously on straight 12V power.
Measurements made with a quality meter.
Using a stable lab power supply at 12.0VDC. Circuit is on only 12 sec. at a time.
The V drop across the fet is 1.6V. At 1.8A full-on current, that's about 3W, but the mosfet doesn't even get warm to the touch.
Why is the V drop across the mosfet so large? The Rds-on is spec'd at only 7mOhm. At 2.5A, should not that give a drop of less than 20mV? Applied gate V is measured at 5.3V. Spec is +/-20V max, typical 2-4V, so fet should be "full on".
Expected to see, and need, almost the full 12V on the load.
Probably reading the specs wrong, but I'm, as usual, puzzled.
Thanks for any help.

 

Specs read correct. What is the source of the 5.3 volt gate voltage?

 

It does sound a bit high to me.
Have you ensured there is no voltage drop in wires and connections involved in your measurements?
Also, if you have an oscilloscope, check there is no oscillations going on as that will upset your readings.

 

It does sound a bit high to me.
Have you ensured there is no voltage drop in wires and connections involved in your measurements?
Also, if you have an oscilloscope, check there is no oscillations going on as that will upset your readings.

Also, is the gate drive adequate??

 

The source of the gate voltage is an Arduino Mega board. Gate V was measured at 5.3V.
The long leads to the protoboard result in a measured V 0f 11.9V when the power supply indicates 12.0V. High quality PS with both V and I digital readout.
Do not have a scope. What factors could cause the problem you refer?
I'm thinking all the probably poor contacts on the protoboard add up in this low R circuit. The V load is measured including the power leads at the connection point to the board.
"Is the gate drive adequate?" Spec says 2-4V typical, have a solid 5.3V. Does the 10K resistor have any effect on the output? I used 1K, 2.2K and 10K, noticed no difference in heater, but didn't make any measurements yet on the lower value R's.
Thank you for the amazingly fast responses.

 

The 10K resistor will be ok as long as the gate voltage is not loaded down by it, and an Arduino can easily drive 10K.
I often use 100K as a pull down because in normal operation, the gate is driven both high and low.
The resistor is a safeguard for having the gate in a defined state at startup as the Arduino will not be driving until the program runs.

 

Have 68K Rs handy, so will use those. Tried one...no change in the large drop across the mosfet. Thinking that 28g jumpers and crummy protoboard pin/sockets are messing with the voltages?

 

Found an answer on the Arduino forum ... the mosfet I used is not a logic level one. Need to find another spec.