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Looking for a low-medium-hi pushbutton switch

Discussion in 'General Electronics Chat' started by unbwogable, Nov 12, 2011.

  1. unbwogable

    Thread Starter New Member

    Feb 11, 2009
    I'm looking to use an LM338 to control a heater for an incubator, and I need a circuit that will allow me to vary the resistor value, adjusting the output voltage as a result. What I'm really after is a circuit that I can push a button and have it 'step' to the next resistor value. I've scoured the internet, and even attempted a few circuits myself, no luck. Anyone have any ideas?
    The heating element draws 8 amps at 12 volts (pulled it from an old heated seat pad).
    I've considered PWM but can't find a way to control it via pushbutton (pshbutton is required so i can control it from two locations, and also so i can have a couple of preset temps)
  2. crutschow


    Mar 14, 2008
    The LM338 is 5A maximum so you can't use it to control an 8A heater. There would also be a lot of wasted heat to dissipate with linear control.

    The most efficient way to control the heat would be with PWM. You could use a 555 to generate the PWM signal and use that to drive a MOSFET power transistor to control the heater current. If you Google "555 PWM" you will get many hits.

    To obtain steps you could use some type of logic sequencer. One way is with a shift-register, controlled by the push-button, which selects an analog switch to connect different PWM duty-cycle resistor ratios to the 555.

    If you need more help with that I can offer some further suggestions.

    Edit: You could also use a Johnson counter for the sequence, such as a CD4017.
    Last edited: Nov 12, 2011
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    A digital potentiometer with up/down inputs comes to mind. But the regulator you mentioned has a 5A output current rating. I don't think it's a good idea to use a linear regulator for such a high current.

    IMO PWM is the right way to go.

    What's your input voltage? Is it stable?
    How many steps do you want?
    Last edited: Nov 12, 2011
  4. unbwogable

    Thread Starter New Member

    Feb 11, 2009
    I have no idea why I thought I could drive 8 amps with a 5 amp reg.

    Pwm is clearly a better option then. I only really need 3 steps + off. Is there such a thing as a tristable multivibrator? Lol

    Voltage is pretty steady. Fluctuates between 12 and 13 volts. Its a low voltage feed from a diesel generator
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Ok here comes a "two IC" solution.

    PWM frequency is about 23Hz. The duty cycle levels are: 0%, 35%, 65%, 100%.

    For now, the command is given when the pushbutton is released, not when it's pressed. If that needs to be changed, tell me. It's because the counter counts when there is a positive going edge on it's input - while the other input is positive too.

    Also, logic at the moment is such as that when on maximum level and UP is pressed again, it will overflow,i.e. it goes back to zero, same is true for the opposite. I guess it's supposed to not accept anymore commands in the same direction, once the maximum/minimum is reached, right?
    I just don't have anymore time today to do this.

    There are inumerous ways to do this, the counter and opamp PWM was just the first that crossed my mind. It could also be done with a 555, a digital pot etc.

    The MOSFET was the first I thought of because it's quite common, you may use anything else that's rated for this current /voltage. It will need a heatsink.
    The 258 is actually not suitable to drive a MOSFET, maybe a driver / transistor needs to be added between those two.

    The two IC's need decoupling capacitors, which I didn't draw.

    Make sure that the current that flows through the load does come directly from the power supply, IC current and load current shall not share the same wires.
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    I forgot one thing. The pushbuttons need to be debounced. ;)
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Ur simplest solution is to use MOSFET with 555 PWM circuit

    Oh I forgot..u need switches.
    How about a mikrochip
  8. bertus


    Apr 5, 2008

    You still can use the 555 PWM circuit of Bill_Marsden.
    On the bottom of this post there is a version that drives a mosfet:
    In stead of R5 (the varable resistor), you can have a couple of fixed resistors to set the powerlevel.
    The total value of the resistors should be 10K (the value of R5).

    For example:
    If you replace the 10 K by 3 X 3K3 resistors, you will have ≈ 0 %, ≈ 33 %, ≈ 66 % and ≈ 100 % available.

    Last edited: Nov 13, 2011
  9. SgtWookie


    Jul 17, 2007
    Simple PWM will provide no control over the temperature of the incubator. If the incubator is for some kind of bird, either domestic or wild, you need a feedback loop to maintain the temperature at the desired setting of 99°F-100°F if you are using a fan to keep the air circulating, or 102°F if not using a fan.

    Otherwise, even small influences outside your control could cause the temperature inside the incubator to vary over a fairly wide range. If the egg temperature range exceeds the above temperature ranges for your type of incubator by just a degree or two for even a short period of time (15-30 minutes), the embryos will not be very likely to survive. If the temperature is below the suggested range, the embryos may survive, but will take longer to hatch.

    You also need to turn the eggs periodically.

    I recently saw a very nice project for an incubator that included a method for turning the eggs, air circulation via a fan, and accurate temperature control using a commercial unit that wasn't very expensive - but I can't seem to locate it at the moment.

    The thing about trying to build a circuit like that yourself for such a narrow range of temperature is that it will be difficult for a novice to achieve the precise temperature control that is necessary. If you insist on trying something like that, you might consider a Velleman MK138 thermostatic control kit, and replace the thermistor with one that has a higher resistance -or- add more resistance in series with the thermistor.
    Link to kit page:
    Assembly instructions and schematic:

    The relay is only rated for 3A @ 24V. However, the kit could be modified/adapted to control a suitable power MOSFET for the heater.

    If you're planning on using an 8A 12V ~96W, that's a fair bit of heat. I guess you're making a rather large incubator?

    One possible problem with using your seat heater is, what will you do for a replacement when it burns out?
    Last edited: Nov 13, 2011