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mosfet problems

I am having trouble with a project and hope someone can help. I have a pwm motor speed controller I bought from Carl's electronics (see http://www.electronickits.com/kit/complete/motor/ck1400.htm). I put this together and it works fine. Then I rigged a n channel mosfet to turn on and off from a rf receiver signal, with just the motor this works fine. Flip the receiver on the motor goes on, flip the receiver off the motor goes off (I used the mosfet circuit in this post from LMR http://letsmakerobots.com/node/4285). Problem is I can't put the pwm circuit together with the rc controlled mosfet switch. I want to be able to turn the motor on and off with the remote and while it is on adjust the speed with the pwm circuit. Eventually I need to be able to stop the motor in a specific position, I was going to tackle that after I got the rc part working. Any help would be greatly appreciated.

I forgot to mention when I wire the rf signal to turn on the mosfet that controls the pwm circuit it turns the circuit on but it won't turn off.


circuit.jpg163.35 KB

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I just checked the datasheet and you're correct the RWS 434 is showing max voltage of 5.5 so I need to fix that. If I run the system without the pwm circuit it works fine however and I can turn a led on and off with it at 9 volts.

By 9 volts I am referring to the little rectangular batteries with clips.


With the battery being a small 9 volt, it has probably saved the RWS 434 from being fried with over-voltage. The small 9 volts do not deliver a lot of current, and so may drop a bit in voltage when trying  to power the 555 circuits, the motor (especially), and the receiver/decoder circuit. There could be any number of things happening from back EMF from the motor to maybe a ground loop somewhere causing the flaky circuit operation. The PWM circuit is producing an oscillation, so it could be putting noise on the power lines that is taking out the operation of the receiver by spiking the supply beyond the 9 volts briefly.

In addition to getting something near 5 volts to the receiver/decoder, perhaps a filter cap or 2 would help. A 0.1 uF for high frequency (near where the receiver/decoder are powered) plus a 100 uF electrolytic for lower power supply filtering might clean up the power supply.

 It might be good to try a simple battery pack, like 4 AA NiMh, to run eveything from. The motor would run much slower, but the batteries should be more than sufficient to supply current to everything. If you wanted a faster motor, you could seperate the supplies and give 9 volts to the PWM side while giving 4.8 volts to the receiver decoder. 

kudos and much thanks to robologist. I got a LM7805 voltage regulator and used it to power the rf receiver. The mosfet switch now works as expected.

Where would I put the filter caps you suggest? You suggest a 0.1 uF, a non electrolytic, somewhere on the power supply to the receiver? and the 100uF where would that go?

thanks so much

Small caps like the 0.1 are typically placed on the power and ground lines near ICs. Just one on the receiver, maybe another near the decoder. The 555s already have some caps nearby. The 100 uF should be near the output of the voltage regulator, the 5 volt and ground pins. Note that this cap is polarized, usually has a strip with minus signs pointing to the ground lead.

All the V+ connections are to a common 9 volt battery, all the grounds are to the negative terminal of the battery. The 5 volt label was left over, those circuits are good up to 18 volts I think. All the grounds on the pwm board are tied to the connection going through the switching mosfet, which when on is intended to ground the pwm board and turn on the motor.

Should I have the pwm circuit separated from the motor somehow and have the pwm circuit always on and the motor only being switched?

thanks for looking


Was also curious as to what you mean by a common 9 volt battery? Little rectangle with 2 snap clips at the top? Or the rechargable 9.6 volt RC packs? 

Not all are 12 volt. The RWS-434 shows a needed operating voltage of 4.5 to 5.5 volts. And since it is suppsed to be operating at that level, the HT-12D should be too, so that it can actually read the On/Off signals from the reciever. Then check your FET to see if it can be switched by a logic level 0 to 5 volt signal.

Not entirely sure if this is the problem, but has a good chance of being the culprit.  

Is the ground from the added MOSFET attached to the ground of the Holtek Decoder IC and the ground of the RWS-434? What is supplying 5 volts to the RWS-434 and HT-12D?  Compared to what is supplying power to the PWM circuit and motor?

I attached a jpg of the circuit, my apologies for the crude nature of it. Essentially I tried to take the connection that would go from the pwm circuit to ground and route that through the mosfet that is triggered by the rf signal. If I slow the motor down a lot it works but as soon as I rev up the speed it locks on. Manually disconnecting and then reconnecting any of the mosfet connections turns off the motor. It never seems to just turn on but once the rf signal is sent and the motor is set to run about 30 percent of top speed or more it latches on.


I just found that if I run the motor fairly slow that the mosfet switch works. As I increase the speed there is a point where the unit won't shut off anymore, is that a clue for anyone? I am trying to put together the schematic so this makes more sense.