Hi

I've been working on my project for a while now and have got the infra red link to work.. thanks to help from people on here....

My new problem is with two capacitors.

If you have a look at this datasheet:

http://www.lsicsi.com/pdfs/LS7225_26.pdf

the last circuit. There is a 4.7uF (electrolytic?) capacitor that when a key is pressed should charge up and hold the voltage high for a period of time untill it discharges at pin 11, so while it is high the rest of the unlocking code can be inputed. The problem is as soon as i release the key the voltage goes low - i think the capacitor isn't charging or im not using the correct one?? im using a 4.7uF electrolytic capacitor at 63V the circuit is running from a 5 volt supply.

The other capacitor im unsure about is the one between the supply and gnd.. the datasheet doesn't show any values, what kind of capacitor should i be using if one is essential here.


Any help here would be much appreciated thanks



Carl

 

The capacitor sounds fine, how do you know the capacitor isn't charging up? - your meter could be discharging the capacitor, particularly if it's an analogue one.

The other thing is the 'anti tamper' circuit, this is designed to quickly discharge the capacitor - is that working correctly? - you could remove the transistor to prove is that's the problem.

You should really include one in any circuit, it provides a low impedance path between HT and ground - which is usually essential.

A 100uF should be fine!.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

It was a presumption that the capacitor wasn't charging up, as i set the circuit up and tested it but it wouldn't work... so i then put pin 11 to +5 volts to keep it high and typed in the rest of the unlock code and it unlocked but then would have to phsicaly move the wire from +5 to ground to get correct operation...

Yes it is a analogue meter but i would have thought it would have registered a positive voltage for a brief second, no?

Yeah thanks for that does the 100uF cap need to be of any particual type though?


Carl

 

Try it on a higher voltage range - analogue meters are usually 20,000 ohms per volt - so on a 10V range it's 200,000 ohms, or on 100V it's 2,000,000 ohms. So a higher range will load it less, and also the needle hasn't got as far to travel (which takes time).

Just an electrolytic - anything will do, as long as it's voltage rating is higher than the supply you're using - use a bigger cap if you have one handy, 220uF, 470uF - whatever is to hand.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

yeah you were right the capacitor was discharging! I don't have the ls7225 connected to a keypad but an eprom, when you said the capacitor was discharging i thought maybe the eprom output pin is sinking the current!? so i put in a diode and that seems to have sorted the problem out along with a larger value capacitor about 22uF so it would stay charged longer.. Thanks for the help!

Although there deffinately is a problem with the tamper circuit as it should stop you inputing the unlock code for about 12 seconds if there has been a few attemps at entering the wrong code but this does not happen will need to check my wiring on the bread board first i think.


Carl