Capacitor Issue

[carlosthejackle2001]

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:

https://www.electro-tech-online.com/custompdfs/2004/04/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

 

[Nigel Goodwin]

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 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.

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.

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!.

 

[carlosthejackle2001]

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

 

[Nigel Goodwin]

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

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).

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

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.

 

[carlosthejackle2001]

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