Need to control physical hot plate knob

[Emm]

I need to turn a physical hot plate knob both ways using some sort of small (I thought toy DC) motor.  The plan is to set up a computer-monitored temperature control on an analog hot plate.  I've some sort of power supply (I'll have to get details later), and I don't want to alter the hot plate in any way -- it's not a dedicated operation.  I also need to do it without a DAQ board, if that's possible. 

Thanks! 

[JÞB]

I'm not really certain that you've got a good concept for your project however, I can suggest a possible solution.

Use a DC motor and gearing to allow the motor to turn the knob SLOWLY. 

Connect the supply to the motor through relays so that 3 states exist : 1)no voltage applied <Stop> 2)pos applied <Turn CW> and 3)Neg applied <Turn CCW>

Control the relays with transistors having their bases connected to the PC Parallel port bit 0: and Bit 1 

Use write port to toggle either bit on for a programmable time to turn the knob CW or CCW.

Interesting experiment-

[AnalogKid2DigitalMan]

Following on Jeff's suggestion, you may want to include limit switches at both ends of knob travel to cut power to the motor in the offending direction. This will prevent stalling the motor in a locked rotor condition, stripping the knob from the shaft, or damaging the heater control.

-AK2DM

[johnd01]

Fallowing on to Analogkid:

place a diode in parallel to each of the limit switches so the motor can be pulled off the switch but not run into the stop.

Instead of putting in 2 relays a motor  2 limit switches and 2 diodes and wiring all that up why not put one bigger relay in the power line and control the on/off time with it. Would be a lot simpler than trying to deal with the heat/cool time constants in adjusting the control which probably turns the power on or off. The relay could be solid state. If you are really trying to calibrate a thermostat then you need the motor approach.

[Emm]

I'm a bit weak on the electronics, which is why I've asked about it, so I don't really understand the "relay".

@AnalogKid, I don't understand how the limit switch works -- is it a physical placement? 

@Johnd01, if you mean in line with the DC motor, okay.  If you mean in line with the hot plate, I don't want to alter the hot plate, and I'm not sure how to plug the hot plate into something else that plugs into the wall's 110V AC. 

I'm not trying to calibrate a thermostat, I only need it to keep it within ±3˚C, so if left unattended for ~30 minutes it doesn't boil my water bath, and it doesn't cool too much and let my lab-mates fats precipitate out of their solutions.

Thanks! 

[JÞB]

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Emm wrote:

I'm a bit weak on the electronics, which is why I've asked about it, so I don't really understand the "relay".

@AnalogKid, I don't understand how the limit switch works -- is it a physical placement? 

@Johnd01, if you mean in line with the DC motor, okay.  If you mean in line with the hot plate, I don't want to alter the hot plate, and I'm not sure how to plug the hot plate into something else that plugs into the wall's 110V AC. 

I'm not trying to calibrate a thermostat, I only need it to keep it within ±3˚C, so if left unattended for ~30 minutes it doesn't boil my water bath, and it doesn't cool too much and let my lab-mates fats precipitate out of their solutions.

Thanks! 

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control to 3 deg C is not going to be easy without modification of the hardware.  (It CAN be done but the math gets really involved)

The manual hotplate is the wrong tool-  you have got to look into a real temperature chamber to achieve this control.  heck- even a small used pizza oven could be calibrated to meet the need in a manual mode.   Time to look at garage sales and ax-man surplus stores if the $ ins't there to buy gear

[Emm]

1.)  I've done this type of control (within ±3˚C) with a hot plate manually.  Not looking to "heat to temp" as much as "keep at temp", but I imagine a "heat to temp" would be easier with some rate dependency.    

2.)  It's the only instrument I have.  It's not my lab - no budget.

There's no problem other than time lost if the temp goes outside the range, I just don't want to have to watch it every second. 

[JÞB]

Alright -

you are going to need a PID control algorhythim to keep the temp in range-  It does just what a human does (Oh its a little bit low so just sooooo much more- ect...) like adjusting the shower.  Unfortunatly YOUR only feedback is a temp probe that you are intefacing HOW?

[Emm]

I figured out how to control the motor with parallel; but came to the same realization about the thermocouple/thermistor connection about the same time.  I don't know if it's possible to connect and read without the DAQ?  

[JÞB]

I've never tried this but it should be possible.--

Tie the thermistor to the feedback of a ring oscillator.  The resistance of thermistor will vary with temperature. Therfore the ring oscillator frequency will change. Your PC probably has a MIC input record the tone and adjust the heat as needed.