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all you ELECTRICAL Engineers out there.......need some help....
- Thread starter heat23
- Start date
You can make such a pushbutton using a spring mechanism and electrically conductive foam. The more you compress the foam, the more current it conducts (the lower the resistance). With the right kind of foam, it could also act as the spring mechanism. The push button would press on one metal contact, and another contact would be on the opposite side of the foam.
A circuit to drive your LED would be quite simple You could do a decent job with the foam, a couple of resistors and a transistor and a power source, such as a battery. Multiple LED's would be only a little more complex. Essentially, it would be a bar graph driver circuit. You can do four LED's with a quad comparator or up to ten LED's with with one chip, such as a National LM3914.
Conductive foam isn't very predictable, because the resistance depends on the thickness and area of the foam, as well as the characteristcs of the type of foam you find. You'll need to know enough electronics to understand how to bring such a ciruit into the desired range of operation and to calibrate it.
A circuit to drive your LED would be quite simple You could do a decent job with the foam, a couple of resistors and a transistor and a power source, such as a battery. Multiple LED's would be only a little more complex. Essentially, it would be a bar graph driver circuit. You can do four LED's with a quad comparator or up to ten LED's with with one chip, such as a National LM3914.
Conductive foam isn't very predictable, because the resistance depends on the thickness and area of the foam, as well as the characteristcs of the type of foam you find. You'll need to know enough electronics to understand how to bring such a ciruit into the desired range of operation and to calibrate it.
heat, it would help if you would give us a better idea of what you are trying to do. There are many ways of attacking a problem, knowing what the problem is may let us suggest a better way of solving it.
From what you have given so far.
You want a transducer to convert pressure to light.
A load cell would be the most accurate(do you need accuracy) but is also expensive and may be difficult to interface.
Same goes for a strain gauge.
One option would be to have a spring mounted under the push button. Link a lever from the push button to a potentiometer to measure the distance that the button was pushed. The lever would turn the pot. changing it's resistance. That would be easy to interface at the cost of some mechanical work. A high resistence pot would give a good change in output for a small amount of travel. I wouldn't use a wire-wound pot, one of the conductive elements would give a smoother response.
I havn't checked but I don't think a LED's light output is linear with respect to current. What do you need the output for? A simple linear current or voltage meter would give a more accurate display. If you want to use LED's a bargraph meter would work well but cost a little more and take some wiring.
Circuit- probably a wheatsone bride with an op-amp.
From what you have given so far.
You want a transducer to convert pressure to light.
A load cell would be the most accurate(do you need accuracy) but is also expensive and may be difficult to interface.
Same goes for a strain gauge.
One option would be to have a spring mounted under the push button. Link a lever from the push button to a potentiometer to measure the distance that the button was pushed. The lever would turn the pot. changing it's resistance. That would be easy to interface at the cost of some mechanical work. A high resistence pot would give a good change in output for a small amount of travel. I wouldn't use a wire-wound pot, one of the conductive elements would give a smoother response.
I havn't checked but I don't think a LED's light output is linear with respect to current. What do you need the output for? A simple linear current or voltage meter would give a more accurate display. If you want to use LED's a bargraph meter would work well but cost a little more and take some wiring.
Circuit- probably a wheatsone bride with an op-amp.
heat23 -- The foam thing is easy. 🙂 You might be able to find some in a local electronic store that will work. Conductive foam about 1/4" thick is used to hold small quantities of IC's in various packages, so they may have some junk pieces lying around. The trick will be to find some that is springy enough and has a useful range of conductivity versus pressure.
Also, I had a senior moment when I went into the bar graph thing. If, by using multiple LED's you meant one lighting after the other, you couldn't do it that way. I have built such displays, but the circuitry is a bit trickier.
etch -- You're working way too hard. The light output from an LED is pretty linear with current, once you get past the dark current. That's the current level that flows through the LED before it starts emitting light.
You can make a very linear LED drive by driving it from the collector of a transistor configured as a current source/sink. Put your control voltage on the base and a current defining resistor from the emitter to ground. The collector current equals the voltage from the emitter to ground, (the base voltage minus about 0.6 volts) divided by the emitter resistor.
All you have to do is make a voltage divider to the base of the transistor, using the conductive foam as either the series or the shunt leg, depending on whether you want the LED to get brighter or dimmer with pressure.
You have to know the resistance characteristics of the foam to select the other resistor for your divider. Way too easy. 😀
Also, I had a senior moment when I went into the bar graph thing. If, by using multiple LED's you meant one lighting after the other, you couldn't do it that way. I have built such displays, but the circuitry is a bit trickier.
etch -- You're working way too hard. The light output from an LED is pretty linear with current, once you get past the dark current. That's the current level that flows through the LED before it starts emitting light.
You can make a very linear LED drive by driving it from the collector of a transistor configured as a current source/sink. Put your control voltage on the base and a current defining resistor from the emitter to ground. The collector current equals the voltage from the emitter to ground, (the base voltage minus about 0.6 volts) divided by the emitter resistor.
All you have to do is make a voltage divider to the base of the transistor, using the conductive foam as either the series or the shunt leg, depending on whether you want the LED to get brighter or dimmer with pressure.
You have to know the resistance characteristics of the foam to select the other resistor for your divider. Way too easy. 😀
Harvey, i don't understand why you say I "am working too hard". I expressed my view that I did not think that LED's light output was linear with current. My HP books are at work, so I cannot confirm either way right now. I will say I don't remember any circuits that are set up to work that way. I have seen circuits that varied the duty cycle or frequency to vary brightness, usually used to reduce the power requirement. Seven segment displays are a good example.
I think the conductive foam you suggested is an interesting idea. My only concerns are that it would not be accurate as to force or repeatable over time as it degrades. My work requires accurate, long-lasting solutions and that is what I lean to when designing.
I think the conductive foam you suggested is an interesting idea. My only concerns are that it would not be accurate as to force or repeatable over time as it degrades. My work requires accurate, long-lasting solutions and that is what I lean to when designing.
From my own personal EE -
<< A old key from an electronic organ, the kind that the harder you hit the
key, the louder the sound.
Alternately, a syringe that is hooked to a pressure sensor that in turn is
read by a PIC that controls the LED (with PWM). (the syringe is filled with
air, and the "button" would be the top of the plunger, the needle side of
the syringe is hooked via tubing. >>
<< A old key from an electronic organ, the kind that the harder you hit the
key, the louder the sound.
Alternately, a syringe that is hooked to a pressure sensor that in turn is
read by a PIC that controls the LED (with PWM). (the syringe is filled with
air, and the "button" would be the top of the plunger, the needle side of
the syringe is hooked via tubing. >>
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