Howdy all, I'm new to the community and I wanted to know the best route (ie kit/hardware) to go for building a pressure sensitive heat pad (this is for a class project). I wanted to program the board and heater pad to cycle for .5hrs on, off, and on again, while maintaining a set temp. I was thinking of a basic starter kit but I was wondering what else I need? Looking at TI hardware for stepping up power (W).
thanks
JT
tarin4950:
Howdy all, I'm new to the community and I wanted to know the best route (ie kit/hardware) to go for building a pressure sensitive heat pad (this is for a class project). I wanted to program the board and heater pad to cycle for .5hrs on, off, and on again, while maintaining a set temp. I was thinking of a basic starter kit but I was wondering what else I need? Looking at TI hardware for stepping up power (W).thanks
JT
Welcome to the Arduino forum. Glad you are interested in the little bugger!
If you want help, you need to be a bit more descriptive of what you want to build. What heat pad? Do you mean one for human use? What does pressure sensitive mean in relation to a heating pad? where is the pressure? From the user? You are not supposed to lay on a heating pad?
If the pad is supposed to cycle on-off, what controls this cycle, a timer? IF it cycles on and off, how does it maintain temp? What powers the heating pad?
Finally, what is TI hardware and how can anything step up power?
A basic Arduino starter kit is a great way to learn before you buy anything for the project.
Paul
Hi JT.
If the heating pad draws 10A of 'house power' (120V/240V) then a cheap (typical $2 or less) hobby relay module can be used to switch power to it simply by turning an IO pin ON/OFF.
Only you must make sure it doesn't switch too often, once a second may be okay but not every second, switching a relay makes heat that must dissipate to not build up. You do this in code by having one part that switches the relay only when a timer lets it and the other code only sends a signal (sets a variable from 0 to 1), the relay code sees that and makes the relay switch after the timer says okay ---
-- so you have 2 (can be more) parts of code doing their own things safely and coordinating through a value in a variable.
-- each part is smaller than the whole and ever only does 1 or 2 small things at any one time, is much easier to debug.
As to pressure/people-presence sensing, you have many choices with some being -very- low cost. Literally, 2 pieces of metal (even a foil gum wrapper works for this) with foam between and some wire and maybe 1 resistor can do this though it'd be different if the foam insulates or conducts but still either can work.
A Piezo disk can work but is actually trickier in circuit and code.
If you have a physics prof you want to make happy (unless too busy!), ask about using capacitance or resistance as means of sensing.
IF the heating pad is similar to mine, it has it's own internal thermostat and switchable heat settings. The op may have to rewire the unit to do what he envisions.
Paul
If it's similar to mine then the lowest setting can be too hot to leave on while sleeping.
I like the pressure sensor part, turns off when not engaged.
thanks for all of the input and (more) technical questions. Here goes:
using an Arduino kit, I wanted to build a heating pad that would turn on when a pressure sensor is activated ie:a persons weight or a strap running across the sensor and around the arm. I had sports related injuries in mind when developing ideas for this project. I would like to be able to set cycles (roughly 30 mins) of on-off-on and power down. I wanted to set a max temp (200F) and include a cut off if that temp is exceeded. As for the power source I'm up in the air about it. I really just want it to work for now so an external wire to 120V is fine or USB. eventually I'd like it to be powered by battery. The heat pad I'm using has a silicone RTV matrix so it can be submerged in water. I did a little testing and found out it is 37 Ohms, and to get the 200 F I set parameters to not exceed 2A, I ended up with about 35VDC at .89A to get about 31W. so I wanted to know what kit would be best to supply these needs? what other hardware should I look for? Im looking on TI.com at the LM2733 for my switching and power needs. I'll admit the software part of this will be a bear for me. I have tinkered a little with the software aspect but I am by no means a pro nor a novice at it.
TI.com at the LM2733 for my switching and power needs.
That will only convert an input voltage to a higher output voltage. The input current will be higher than the output current. The power, that is the voltage times the current, of the output will be lower than the power of the input. You need a lot of skill to turn that chip into a workable device. I have managed engineers who specialised in such designs and have never met one that could make one work first time, and you do need to use a PCB, you can't bread board such a thing.
I really just want it to work for now so an external wire to 120V is fine or USB.
USB can only supply a maximum of 2.5W so I don't think you have a choice here.
You can switch many amps of up to 60V right off an Arduino pin with a IRLZ44N MOSFET. I bought 60 on eBay for less than $10 (paid 14.5 cents each), a single one would have been closer to 30 cents.
The Arduino is used to switch the FET, the IRLZ44N is best used to drain power to ground with power always being available in "the high side" and current only flowing when "the low side" is switched open to ground.
There are many other MOSFETs that can do the job but get ones that switch on TTL level signal, the norm is 10V to open full.
Do NOT use the FET as a valve. It will get hot. As a switch it is very efficient, as a valve it's a waste of silicon.
Best cheap Arduino-able electronic thermometer I know of is the 18B30. It uses the 1-wire (or 2) interface and library.
With the FET you pulse power through the heating pad using a PWM (Pulse Width Modulation) pin or just switch a pin on and off with code. You can very closely control that pad.
Look into cheap Peltier wafers if you want a cooling element to alternate with the heater. Peltiers move heat.
as with all projects. the sensor is the key.
you need to figure out what sensor you can use.
you could use a pressure switch or a pressure sensor.
one is digital on/off the other outputs an analog signal
once you have that, bob's your uncle.
everything else is moot until you can sense there is a user present.
Back again. I got a couple more items to ask about. I was thinking of putting the heat pad in line (series) with a push button switch. could I put the switch in line with a step up converter and then to the heat pad? would I need any specific code to make it function? The idea, for now, is to put everything in series-to see if it works at all-and then add in a temp sensor (still need the coding for this part). I'm searching around the site to see if there is any code I can use. Thanks again for the guidance and comments.
JT
Using a step up converter with a heat pad is just silly. What causes the heat is the current. High current step up converters are expensive and the cheap ones on eBay do not provide the headline current figures. You need to get or make a heating pad that matches the voltage / current you have.
Yes for testing you can just switch the power to the heater. But in the end that switch needs to be one tha you can control by an Arduino.