Use 74LS instead 74HC and 74HCT

[Tioleco]

I'm going to mount this prescaler but I only have of LS series , can I use it instead of HC and HCT?

[ratdude747]

Re: Use 74LS instead 74HC and 74HCT

Sure. The power consumption will be somewhat higher (but less so at high frequencies) but since you're already running linear 5V, and with comparable switching speeds, I say try it.

[stj]

Re: Use 74LS instead 74HC and 74HCT

no, read your datasheets.

HC is not 100% compatable with LS, HCT is.
and all 3 have different maximum speeds and drive handling.

if speed is a problem look at AHC / AHCT series.

[eccerr0r]

Re: Use 74LS instead 74HC and 74HCT

I don't think speed will be a problem, but likely you will run into a power problem - The PP3 battery and the 78L05 will be woefully underpowered with all LS devices, but using the AC/HC/HCT devices it will be fine.

[redwire]

Re: Use 74LS instead 74HC and 74HCT

The output voltage of the first pre-scaler (divide 128) may not be enough for 'LS02.
There is an error on the schematic (100R and 2k2, probably missing capacitor there).
Anyhow, the 1k and 2k2 resistors may need to be changed. The uPB1507 only data is "1.2Vpp output" but no idea of output impedance, so it might not drive the 'LS02 sufficiently.

[eccerr0r]

Re: Use 74LS instead 74HC and 74HCT

It says it's an emitter follower output... which make sense why a pull down is added. Vp-p seems to imply that's peak to peak voltage/swing voltage and not absolute voltage.

If it only gets to 1.2V from GND, that's not enough to even flip a 74HCT02 @5V, granted that cap there will help temporarily. The Fujitsu datasheet says it can source up to 10mA but 1.2mA recommended. A 74LS02 only needs 100µA for a high, so that should be fine...

[redwire]

Re: Use 74LS instead 74HC and 74HCT

Emitter-follower output in uPB1506 cannot sink (any) current for an 'LS02 gate input to go low. That logic family needs -1.6mA and <0.8V or a max. 500 ohm pulldown to work.
For a logic "1" you need >2V at 40uA which is nothing, LS inputs float high.
OP will have to change the resistor values and see if the pre-scaler puts out more than 2V. Not sure how this works at 27MHz.
'HC02, 'HCT02 datasheet

[eccerr0r]

Re: Use 74LS instead 74HC and 74HCT

Another thing that should be changed is that the original schematic wants both inputs tied together, might work better if one input is simply tied low instead, so you don't have to sink as much current. Yeah likely the biasing resistors may need to be changed a bit, but I think it should be possible to solve.

If you need to swap devices, perhaps leaving the 74HCT02 would be best so you don't have to rebias, and the others should be OK to swap to 74LS161. You'll be near the limits of the switching speed of the LS161 if passed a 3GHz signal, whereas the HC161 has some breathing room.

[Tioleco]

Use 74LS instead 74HC and 74HCT

The original design uses a MC12079 for input.

[sam_sam_sam]

Re: Use 74LS instead 74HC and 74HCT

Here is my two cents worth to this

If I was doing this project———>

I would use the exact parts that it calls for because otherwise it might not work the way it should

Let us know how this works when you are finished with this project

Thanks

[japlytic]

Re: Use 74LS instead 74HC and 74HCT

74LS/74HCT runs on 5V while 74HC can run on on 3-5V.