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mahaju
Joined: 17 Mar 2007
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 27 Aug 2008 12:09 Please help with project: Electronic Trainer Kit |
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HI
I am thinkling of building an electronics trainer kit for my final year project. Does anybody have any suggestions? Please reply.
Thanx in advance
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electronrancher
Joined: 24 Mar 2002
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| 28 Aug 2008 21:52 Please help with project: Electronic Trainer Kit |
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How basic do you want it?
Some very low-level things may be:
RC circuit - when they let go of a pushbutton, they can see different RC take different time to reach a comparator threshold and light an LED.
By using a circuit to discharge the RC's cap when the comparator trips, they could make a sawtooth oscillator.
By changing the resistor to a current source, and using another current source for the cap pulldown (both controlled by comparator with hysteresis) they could make a triangle oscillator.
Another basic thing would be negative-feedback op amp. They could change the feedback resistors to produce different gain, amplifying a 1v signal up to 5v or 10v. Or even use a potentiometer to show how gain can be used for volume control.
I mean, there are lots of ideas that can "wow" somebody that doesn't know anything about electronics but is interested. But for someone who is already a junior-level student, the projects should be more sophisticated such as a crude 2n2222 bandgap, or a voltage regulator, or a PWM control circuit. A neat PWM control tries to keep a motor speed constant as the user grabs the spindle to apply load.
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mahaju
Joined: 17 Mar 2007
Posts: 47
Helped: 4
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| 09 Sep 2008 17:34 Re: Please help with project: Electronic Trainer Kit |
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Thank you very much electronrancher for your suggestions. Although I would prefer the kit to be usable by a person of any level of knowledge in electronics, I would be more interested in a more advanced design, as this is also my final year project. Do you have any ideas about implementing PID control or at least P or PI in hardware? Are there any other types of control systems that I may try to find out about?
Thanks.
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electronrancher
Joined: 24 Mar 2002
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| 09 Sep 2008 18:11 Please help with project: Electronic Trainer Kit |
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Motor control would be an easy one to see and physically interact with. Choose a motor with tachometer, and the control loop can change the supply to the motor in order to keep the speed constant. The kiddies can try to grab the motor spindle to slow it down, but your circuit will increase motor power to always keep the speed constant.
You can shoot for PID if you want to, but I have to say that you yourself should know the control method front to back before attempting to build a project to teach others about it. Let's move on to the dirty details of one simple control method.
A lot of tachometers are just a simple disc spinning with the motor. The disc has holes punched in it that an LED shines through. A photodetector outputs a pulse each time a hole passes by, so the frequency of the pulses changes as the motor speeds up and slows down.
Simplest control would be to take those square-wave pulses and use an R-C filter to generate an analog signal proportional to motor speed. Then, you just need an integrator (built using LM13600 or even op-amp type II compensation) to integrate the difference between desired speed (a fixed 1v or something) and the current motor speed (the filtered tach signal).
Now you have an integrated error voltage that can be used to control the motor speed. If motor is too slow, error voltage will rise at a rate proportional to the difference between desired and actual speed. If motor is too fast, error voltage will fall. If motor speed is exactly right, error voltage will neither rise nor fall to keep the motor at exactly the same driving point.
Driver could be a constant-current circuit (just use a V to I converter from the error voltage) or even PWM control.
For PWM, compare the error voltage to a sawtooth ramp, and close a switch in series with the motor when the compare is high. If the error voltage is higher, we will give larger ON pulses to the motor and vice versa. Your PWM can run at 50kHz or so to void making an audible noise.
The motor will be very inductive, so you don't need a seperate inductor in the pwm stage.
I used this scheme to control the speed of a motor for a gemstone faceting machine. It worked fine, you could really press down hard on the grinding wheel and watch the PWM increase but the motor speed stayed dead on at wherever you set the "speed" potentiometer.
Good luck!
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mahaju
Joined: 17 Mar 2007
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