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Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
just when i thought i had mastered simple things like 555s...
im using 555s as simple vcos, modulated by the output of an accelerometer. I have two types of 555s, the tlc low power version, and the ne regular version.
When I use the tlc, everything is happy. when I use the ne, the modulation is GREATLY reduced. In fact, I know my control signal swings about 1V around 2.5V on the control pin, but when I use the ne555 and watch the pin voltage, it barely moves a few mV. If I pull the chip out of the socket, and watch the voltage, everything is normal.
Anyone know what causes this? I checked the datasheets for the various types and didnt see anythign of interest.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
At a guess; the Control Voltage is determined by an internal resistor network, maybe the tlc version uses higher value resistors than the NE version for low power consumption, the lower resistance of the NE may be loading your vco voltage source?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
It is most likely a FET input, and therefore the input resistance will be extremely high, often in the gigaohm (or sometimes teraohm) range.
Edit: Ignore what I said, it seems to be connected to some resistors of unspecified value inside the chip, so the input resistance is controlled by these resistors.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The TLC555 datasheet doesn't say. :-<
It seems to use MOSFETs as "resistors", so the value probably varies hugely depending on temperature, supply voltage, control input voltage, phase of the moon, who was on the Letterman Show the night before etc. But I'm guessing it's always a good deal higher than 6k.
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so embarassing, 555 problem
