Help with 74HC/LS04 inverter

[carlosthejackle2001]

Hi,

I have designed a circuit, the diagram shows a bit with the area i have concern with... what i read about the HC TTL and the LS TTL they are directly compatible... From testing this idea i found that they are not or more the case i am doing something wrong!

When the 74HC04 is in circuit the connection between the inverter input and the tamper o/p (pin 5) from the LS7225 the voltage is aprox 0 but when i replace it with the LS the input voltage rises to 1.21 aprox.

I have tried placing a pull down resistor of 220Ohms but then for some reason the pulse doesnt seem to work!? The pulse according to the LS7225 data sheet is aprox 15 u seconds.

any help would be appreciated and if you need to see more of the circuit i can draw more... (not the best quality sorry)

The cicuit does work but i would like to use lower current, but not use cmos because that would just complicate things having to use buffer circuits.


Carl

 

[carlosthejackle2001]

sorry having problem with attachment

 

[ljcox]

Carl,
What is the LS7225? The HC series is not TTL, it is CMOS.

I don't understand your point "i would like to use lower current, but not use cmos because that would just complicate things having to use buffer circuits"

What are you trying to do? If we knew the full picture, we may be able to help more.

Also, I suggest you delete one of the attachments as they appear to be identical.

Len

 

[Roff]

I looked at the **broken link removed**. The tamper output has an open drain p-channel output stage, which means it can only source current, and it can't source enough with a 5 volt supply to drive the pulldown you need for a 74LS04. Use a 74HCT04 (preferred) or a 74HC04, with a 100k resistor from the tamper output to GND.
What is the purpose of the 12k resistor and the 0.1uF capacitor? The output is too wimpy to drive that. Get rid of both parts.

 

[carlosthejackle2001]

Sorry yeah i just assumed (because of the part number) it was ttl when yes just checked it out, it is indead cmos...

The ls7225 is a chip to help create digital lock device's, i have made a keypad entry system. When you press a wrong key, the tamper output pin (5) supplies a short pulse and i am trying to invert it so it can trigger a 555.

When the 12k and 0.1uF aren't there the pulse seems to have no effect (i.e. doesn't get inverted), i belive the job of the resistor and cap is to stretch the pulse (propagation delay), either way it only works when they are in place.

I will get hold of a HCT and see if that improves things.. thanks



Carl

 

[Roff]

The HC04 should work with the 100k pulldown resistor. If you go much smaller, the HCT04 might be required. If you go below 50k, there is no guarantee either will work, as they only guarantee 50ua of sourcing current when VSS=5v.
Just as a point of interest, I think this part may be PMOS, not CMOS. If it were CMOS, they would have N-channel pulldown transistors on the outputs, in addition to the p-channel pullups.
It might be just as easy to invert the pulse with a transistor. Connect a 10k resistor from the tamper output to the base of a 2N3904 (or your favorite small-signal transistor), 100k from the base to GND, emitter to GND, 10k from the collector to +5V, and connect pin 2 of the 555 to the collector.

 

[ljcox]

Carl,
Yiou did not answer my question:- "What are you trying to do? If we knew the full picture, we may be able to help more"

I suspect that you can do what you want with only one IC (a Hex Schmitt) ie. you don't need the 555.

Len

 

[carlosthejackle2001]

ok

well the pulse when inverted triggers the 555 in monostable mode, this gives me the ability to set the pulse length, the extended pulse is applied to the base of a transistor, the transistor conducts and discharges a capacitor.


The capacitor charges up when the first digit of the code is entered, it will then slowly discharge over time but while it is charged it allows you to enter the rest of the unlock code. however if a wrong key is pressed i need the capacitor to be discharged, this is where the tamper output pulse is required.

The pulse is to "wimpy" to operate the transistor to dishcharge the capacitor quickly!


i think that answers your question?!



Carl

 

[ljcox]

Carl,
Yes it does, thank you.

Attached is a suggestion. I suggest you add a second inverter (there are 5 spares) so you can drive a NPN transistor.

I have eddited the diagram since I did not check the source current capability of the LS7225.

Len

 

[carlosthejackle2001]

Thanks for that, its much appreciated...

I will give it a try when i get hold of a schmit, hopefully it will work as this will save me a fare few components.

Carl