Ok... I am working on a rather simple circuit to produce a heartbeat... nothing particularly special just a 4017 using a 555 timer for the clock. What I need to do is get the timer to be a certain speed at normal temp (set by a pot) and when a device a thermistor or the like is connected to gets warmer it will cause the Timer to speed up to look like the heart is pumping faster.. I just cant figure out a decent way to get a notable change of speed.. I have some very small Thermistors off of some PC Temperature display devices. I am not sure of the 25c temp as I have no controlled way to find out. But anyway.. Yeah I need a little help if possible.. If needed I can draw out what I have but I just need the timer circuit that will vary speed with temperature variance. Thanks..
Y
Yevgenip
- Jan 1, 1970
- 0
What is a Thermistor? You can change the speed on the 555 by changing the value of one of the resistors or the capacitors. I guess that in your case, it would be easier to change the resistance.
It would be helpful if you describe the Thermistor (i.e. What is it's output?)
It would be helpful if you describe the Thermistor (i.e. What is it's output?)
What is a Thermistor?
A Thermistor is a Thermal Resistor where as its resistance is determined by the themperature around it either negtively (i.e. the hotter it is the lower the resistance) or Positive (i.e. the hotter it is the higher the resistance)
I will post a drawing of the schem as soon as possible as I havent done it yet and I will be leaving for a meeting in just a few. Thanks...
A Thermistor is a Thermal Resistor where as its resistance is determined by the themperature around it either negtively (i.e. the hotter it is the lower the resistance) or Positive (i.e. the hotter it is the higher the resistance)
I will post a drawing of the schem as soon as possible as I havent done it yet and I will be leaving for a meeting in just a few. Thanks...
Y
Yevgenip
- Jan 1, 1970
- 0
So which one do you have, the positive or the negative?
Sorry for the late reply lol but it being Christmas and all :
....
anyway.... Yes I was putting the Thermistor in the RC circuit
Its a NTC Thermistor commonly found on Computer Thermal Probes such as a CompuNurse and such.. Thin film....
Anyway... Here is the general schematic I just came up with cause I had the parts on hand... it will basically produce a heartbeat like pulse from the two arrays of LED's
Bump... Bump................Bump... Bump...................Bump... Bump..... Ect....
Anyway... What I am trying to do is have it at around a normal heartbeat maybe slightly slower than a human heartbeat... Which is around 80K or so on the RC... I listed 47K as well thats what I was using when I didnt have the pot in... But anyway... I want it to go from that slow beat progressivly faster with heat which will be from a thermistor connected to the Heatsink of a PC Processor...
To give the effect of "More Stress"......
Any suggestions?
View attachment 36386
....anyway.... Yes I was putting the Thermistor in the RC circuit
Its a NTC Thermistor commonly found on Computer Thermal Probes such as a CompuNurse and such.. Thin film....
Anyway... Here is the general schematic I just came up with cause I had the parts on hand... it will basically produce a heartbeat like pulse from the two arrays of LED's
Bump... Bump................Bump... Bump...................Bump... Bump..... Ect....
Anyway... What I am trying to do is have it at around a normal heartbeat maybe slightly slower than a human heartbeat... Which is around 80K or so on the RC... I listed 47K as well thats what I was using when I didnt have the pot in... But anyway... I want it to go from that slow beat progressivly faster with heat which will be from a thermistor connected to the Heatsink of a PC Processor...
To give the effect of "More Stress"...
... Any suggestions?
View attachment 36386
Last edited by a moderator:
Hi Twisted,
Your timer is not complete as drawn here, if you take a look at this page http://www.williamson-labs.com/555-circuits.htm you may find the solution. Second picture from top shows Ra and Rb and how the decide the charge and discharge time for the capacitor.
Your timer is not complete as drawn here, if you take a look at this page http://www.williamson-labs.com/555-circuits.htm you may find the solution. Second picture from top shows Ra and Rb and how the decide the charge and discharge time for the capacitor.
Not sure why you say its not complete as it is... but I will look into that page... the thing is I need this to change relitive to the temperature of the Computers CPU Heat Sink with no physical tuning by me.... which is why I wanted to use a Thermistor which I seen nothing on that page that covered that unless I looked over it....
OK, as referred to in my last post Ra and Rb decides the ON and OFF periods. You can chose to exchange Ra or Rb or a part of Ra or Rb with the NTC to control the off or on time by temperature changes. The reason your timer is not complete is; there is no path for charge/discharge current for the capacitor if you want
I tired to put a NTC Thermistor in series and in parallel with the Resistor on the RC to achieve this and it did not work out as I thought it would. As far as the Timer Cap discharge the Timer works fine just as I have it layed out and pulses the 4017 Cmos just as it should.. not sure what I am doing wrong with it.. Anyway you could draw me out a simple sketch of what you are refering to though I may be missunderstanding you on the possitoin of the NTC...
audioguru2
- Apr 6, 2004
- 12,026
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Hi Guys,
The 555 works fine when connected with just a single timing resistor to its output. It makes the timing capacitor's charge and discharge times the same, producing a symmetrical output, especially with a Cmos 555 and if the value of the timing resistor isn't too low. :
Twizted's schematic doesn't show the 555's output connected to the 4017's Clk pin 14. ???
The desired effect is to have the 555's frequency increase a lot when the temperature of the thermistor (NTC) increases a small amount. Therefore the resistance of the thermistor must decrease a lot when the temp of the computer increases. Thermistors don't change their resistance enough for this application. Using the thermistor in series or parallel with the timing resistor further reduces its range of change.
Twizted, try using the thermistor by itself as the only timing resistor to see how little its resistance value changes with temperature change. You may need to change the value of the cap to match the resistance of the thermistor for the 555's frequency to be what you want. I think you will find that the frequency changes very little with temperature changes even from freezing to boiling. :-\
I would amplify the resistance change of the thermistor (or even just an ordinary forward-biased silicon diode) with an opamp and use the amplified voltage change to control the frequency of a VCO such as an LM331. The LM331 would replace the 555 as the 4017's Clk oscillator. ;D
The 555 works fine when connected with just a single timing resistor to its output. It makes the timing capacitor's charge and discharge times the same, producing a symmetrical output, especially with a Cmos 555 and if the value of the timing resistor isn't too low. :
Twizted's schematic doesn't show the 555's output connected to the 4017's Clk pin 14. ???
The desired effect is to have the 555's frequency increase a lot when the temperature of the thermistor (NTC) increases a small amount. Therefore the resistance of the thermistor must decrease a lot when the temp of the computer increases. Thermistors don't change their resistance enough for this application. Using the thermistor in series or parallel with the timing resistor further reduces its range of change.
Twizted, try using the thermistor by itself as the only timing resistor to see how little its resistance value changes with temperature change. You may need to change the value of the cap to match the resistance of the thermistor for the 555's frequency to be what you want. I think you will find that the frequency changes very little with temperature changes even from freezing to boiling. :-\
I would amplify the resistance change of the thermistor (or even just an ordinary forward-biased silicon diode) with an opamp and use the amplified voltage change to control the frequency of a VCO such as an LM331. The LM331 would replace the 555 as the 4017's Clk oscillator. ;D
audioguru2
- Apr 6, 2004
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Hi Ante,
No, I think a thermistor is any resistor that changes its value with temp changes, even an incandescent light bulb.
Most thermistors change value very little with temp change, measured in parts per million per degrees C.
BTW, I just got back from the store where I bought my new computer. They said they sold out of it shortly after my purchase.
I got a nice big refund today 'cause I printed a competitor's ad that had a much better price and my store guarantees to match other store's prices! Good timing. ;D ;D
No, I think a thermistor is any resistor that changes its value with temp changes, even an incandescent light bulb.
Most thermistors change value very little with temp change, measured in parts per million per degrees C.
BTW, I just got back from the store where I bought my new computer. They said they sold out of it shortly after my purchase.
I got a nice big refund today 'cause I printed a competitor's ad that had a much better price and my store guarantees to match other store's prices! Good timing. ;D ;D
audioguru2
- Apr 6, 2004
- 12,026
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Hi Ante,
I've never used a glass bead thermistor, just diodes and light bulbs.
I walked all over my computer salesman when I got my "price match" refund. He didn't think his low price could be beat by so much!
I still have many mail-in rebate coupons to send. When the rebate cheques arrive and I sell the nice photo printer (with colour display) the new computer came with, I'll almost make some money by "purchasing" this new computer, a 2.93GHz P4 with a very nice 17" LCD monitor.
What am I going to do with its massive 512MB of Ram and its huge 160GB hard drive? It even has a CD burner and a dual-layer DVD burner.
I wish its Norton AV and SpySweeper wouldn't keep popping-up so often boasting about the good jobs they are doing.
Speaking about thermistors, the fan on my new computer changes its effort with the activity of the computer. Usually it is very quiet.
I've never used a glass bead thermistor, just diodes and light bulbs.
I walked all over my computer salesman when I got my "price match" refund. He didn't think his low price could be beat by so much!
I still have many mail-in rebate coupons to send. When the rebate cheques arrive and I sell the nice photo printer (with colour display) the new computer came with, I'll almost make some money by "purchasing" this new computer, a 2.93GHz P4 with a very nice 17" LCD monitor.
What am I going to do with its massive 512MB of Ram and its huge 160GB hard drive? It even has a CD burner and a dual-layer DVD burner.
I wish its Norton AV and SpySweeper wouldn't keep popping-up so often boasting about the good jobs they are doing.
Speaking about thermistors, the fan on my new computer changes its effort with the activity of the computer. Usually it is very quiet.
Yeah I spaced my timer connection to the 4017.... I will look into that LM331... The Thermistor I have from the thermal probe ranged from around 50K down to around 3K from ice cold around body temperature... If I had a way to lock it at around 90K and it would lower to around 30K I would be ok using it and keeping the circuit simple... but locking it into that range is kinda beyond me at the moment lol.... I will just have to keep tinkering.....
audioguru2
- Apr 6, 2004
- 12,026
- Joined
- Apr 6, 2004
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Hi Twizted,
Your thermal probe has an enormous range to change its value from 50K when freezing to only 3K at body temperature. You just need to know its resistance at the temps that you are going to use it.
Try your circuit with the thermistor in series with a 1K resistor replacing the 47K timing resistor. Since the total resistance is now much lower, the timing capacitor's value should be increased so replace the 1uF cap to about 10uF.
Your thermal probe has an enormous range to change its value from 50K when freezing to only 3K at body temperature. You just need to know its resistance at the temps that you are going to use it.
Try your circuit with the thermistor in series with a 1K resistor replacing the 47K timing resistor. Since the total resistance is now much lower, the timing capacitor's value should be increased so replace the 1uF cap to about 10uF.
