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Hi all
I have a secu-16 and connected to the input I have a very basic light sensor circuit. The circuit is made up of a 1M ohm resistor and a LDR in series with 5V across both components, the input to the secu-16 is taken from the junction of the resistor and LDR. Before connecting the circuit to the secu- 16 I tested it with a volt meter connected to the intended input and it seemed to be fine. The voltage rises when dark and drops when light (depending on which way round the 5V is connected). The problem is though when I connect to the secu-16 the voltage increases by around 1V. So in other words, I measure the voltage before connecting and it may measure 1.6V, for example, but as soon as I connect to the secu-16 it will increase to 2.6V. To test the secu-16 I have another analogue input which is a potentiometer again with 5V across it and the input to the secu-16 is the potentiometer wiper. The same test as above was done but this seems to be ok. In other words I measure 4.1V when not connected and about 4.15V when connected so very little change. Also I swapped the two inputs to test the individual inputs and it made no difference, the LDR circuit increases by about 1V, so not so good and the potentiometer circuit increase less than 0.1V, so very good. Anybody any ideas why this happens as it does become a problem as when the circuit is connected to the secu-16 and is getting dark the voltage is reaching 5V before it is completely dark so I lose a bit of my range. Also, a smaller problem, when very bright the voltage never goes below about 1.5V so although this doesn’t bother me it is the extra bit of range I need at the other end of the scale. Another point to make is that when the voltage on my circuit is above 4V, when not connected the secu-16, it never goes above its maximum of 5.3V when connected to the secu-16.
I read on other post that the voltage across the input when nothing is connected should be 5V but mine reads 5.3V, hence my comment about the maximum being 5.3V above, probably not a problem but wondering if this is ok.
Thanks for any help
Edited by tom21 (07/01/09 08:47 AM)
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