Broken DS18B20

J

Joeroddis

Member
Hello everyone, laying in the bottom of my box of 'useful bits' i found a little sealed package with DS18B20 written on it, i found out that this was the digital temp sensor.

I decided to have a mess about but i can't get it to work, i think i may have fried it. I have it connected the right way for sure and i've got a 4k7 resistor between Vc and adc2 (picaxe 18a) .

Basically i have coded it basically to say turn output 6 on for so long when the temp gets above a certain point. The ouput is connected to a monostable, which has an LED on it's output. Problem is, that this output is always on. I tried changing the program so that op6 would only come on if b2 > 220, but it is still always on, this is impossible being as the thermometer is cool to the touch.

Even more strange is that when i connect my multimeter beteen ground and vcc pin i get 5.08 volts, when i connect my multimeter between ground and the ouput pin i get 5.05v. i suppose this would explain why the monostable hasn't turned off yet, but why is the chip always outputting 5.05v. Surely if i had broken the chip, it wouldn't show anything across the terminals.

Any ideas?
 
Y

ylp88

Senior Member
The DS18B20 is a digital temperature sensor, not analog, and therefore is not read using the PICAXE's ADC functions. The voltage you are measuring is probably due to the 4.7k pull-up resistor on the pin, which as you state correctly, is required. You've connected the device's output to an ADC pin but do note that you should be using the READTEMP/READTEMP12 command to use this device.

ylp88
 
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J

Joeroddis

Member
Yes it is now working, but looks like i'm gonna have to rethink my design :| the 750ms delay is too long. anyway thanks for the help ylp88
 
hippy

hippy

Technical Support
Staff member
One option is to put an 08M between the DS18B20 and your master PICAXE. The 08M can issue a READTEMP and be ready to send a result to the master when told to using SEROUT/SERIN, having sent its data the 08M issues another READTEMP and waits to be askedagain.

That won't get round only being able to get a data update every 750mS but it does mean there's only a very short delay between asking for it and getting it, and the master isn't held up waiting for READTEMP itself.
 
J

Joeroddis

Member
Hey, thanks for the reply, i actually don't have any 08m but i have got about 8 18a and 18x and one 28x. never found need for one until now.
I have built the circuit on the same board but just used an 18a to do the readtemp
 
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