Replacement for U6 & U7 MC14538BCP

[KRinAZ]

For those of us who want to be able to set the primary sample delay below 10uS (I want to adjust it down to .1uS) it's time for a replacement for the MC14538BCP.

Besides this chip is obsolete at sites like Mouser.com so for the next rev of MPP it's time for replacement.

I haven't found a pin compatible faster replacement so far, but going forward it appears a chip like TI's CD74HC4538E would be a good choice.

By my calcs from info on the datasheet, it can generate pulse widths as small as 35nS, but will require minor changes to the circuit and the circuit board.

I vote for this chip (several manufacturers make it) - thoughts? - thx!

[KRinAZ]

Here are some basics about this chip: it is a dual monostable precision multivibrator, 16 pin CMOS PDIP, high noise rejection & low power, supply range from 2 to 6 volts, output pulse width in seconds is .7*Cx*Rx, Rx minimum is 5k & no limit on Cx, so Rx of 5K & Cx of 10pF gives a pulse width of 35nS, it is inexpensive and in stock at all online sources, so it seems like the perfect successor to the MC14538. I've got a couple on order & will be wiring it into my RevC MPP board soon...hoping everyone agrees on switching to this chip (including Qiaozhi of course haha) & posts some replies...

[sinclairuser]

@ george- the mc14538bcp used in the minipulse plus goes down to 10us,i have in stock some mc74hc4538a(ceramic) its pinout is identical.
it is in fact the same chip its lowest setting is 1us,now if i use this chip 0v may not corespond to 10us and it will be harder to setup but should enable the MPP to go sub 10us(in theory as 10us is the limit of the mc1453).
do you think using a chip with a wider adjustment range will need any other considerations other than the setup issue?, i have six of these chips and no real use for them, the specs are so close it would be a shame to buy chips if these work or even work better, what do you think?.
@ satdave uk- the g suffix is for lead free.

i copy and pasted that from #57,page three, in replies to minipulse plus, the above chip is more common i tried one and adjusted it to work within normal parameters.
check out the data sheets it may help your goal, its a variant of the chip george chose and is pin compatable
thanks, aly

[KRinAZ]

@ george- the mc14538bcp used in the minipulse plus goes down to 10us,i have in stock some mc74hc4538a(ceramic) its pinout is identical. it is in fact the same chip its lowest setting is 1us,now if i use this chip 0v may not corespond to 10us and it will be harder to setup but should enable the MPP to go sub 10us(in theory as 10us is the limit of the mc1453).do you think using a chip with a wider adjustment range will need any other considerations other than the setup issue?, i have six of these chips and no real use for them, the specs are so close it would be a shame to buy chips if these work or even work better, what do you think?.@ satdave uk- the g suffix is for lead free.i copy and pasted that from #57,page three, in replies to minipulse plus, the above chip is more common i tried one and adjusted it to work within normal parameters.check out the data sheets it may help your goal, its a variant of the chip george chose and is pin compatablethanks, aly

Thanks for your reply sinclairuser, my understanding is that chip is also obsolete - according to mouser.com - & for me anyway it's a priority to get below 1uS. I personally think we should switch to a chip in normal production & that does away forever with minimum sample pulse width limitations - my 2c

[Qiaozhi]

The MC14538BCP may be obsolete, but TI have a replacement: CD14538BE. There are 5.672 in stock at Farnell UK (Element 14), order code: 1470900.

You could also try the CD4098B, which is pin compatible, but I don't know its minimum pulse width.

There's also the LTC6993 that can generate from 1us to 33.6 secs.

[KRinAZ]

The MC14538BCP may be obsolete, but TI have a replacement: CD14538BE. There are 5.672 in stock at Farnell UK (Element 14), order code: 1470900.

You could also try the CD4098B, which is pin compatible, but I don't know its minimum pulse width.

There's also the LTC6993 that can generate from 1us to 33.6 secs.

Thanks Qiaozhi, I didn't consider the CD14538BE or the CD4098 because both of their minimum pulse width wasn't in the data sheet, and they have a slow output transition time (100nS to up to 200nS per transition, meaning a 0nS pulse width could consume 400nS just in transition times - this vs the CD74HC4538E at 19nS per transition - and I'm interested in increasing performance at the very low TC end of the spectrum.

Those two would be an improvement over the MC14538BCP though - so again thanks.

I am going to go forward with the CD74HC4358E's for performance, just another of my mod's I guess...

[vbeeeks]

Will 74VHC123A fit the bill?

[KRinAZ]

Will 74VHC123A fit the bill?

Yes, it isn't pin compatible with the MC14538BCP either but once wired in would work well. Since it is quite similar to the CD74HC4358E and I have those (CD74HC4358E's) on order already I will go with the CD74HC4358E - thx

[aries1470]

Hi, just thought to ask if you have any updates for the HC model. Thanks.

[KRinAZ]

Hi, just thought to ask if you have any updates for the HC model. Thanks.

Hi aries1470, yes & thx for asking. I installed the CD74HC4538E in place of U6 only, left U7 alone, was easy as only 3 pins are different & the connection to the RC is slightly different. I was able to solder 2 components to the chip leads on top of the chip & plug it into the U6 socket. I'm camped at a mining claim at the moment, as soon as I get back & can get to my notes I'll post with all the details so anyone can install a CD74HC4538E for U6. It certainly allows a sub 1uS delay depending on the RC values.

[aries1470]

Hi KRinAZ,
Thank you for the update, can wait for the pics and scehmatic+values for how the connections are made

edit: I have found a large source of 74HC4538 here in Melbourne, Australia, plus CD4538BCN made by Fairchild, HEF4538BP by NXP, and 74HCT4538N. That is why I was inquiring about possible mods.

[KRinAZ]

OK, no schematic or pics at this time - but I really don't think they are necessary - these instructions are simple to follow:
Comparing the MC14538BCP Datasheet MC14538BCP.pdf to the CD74HC4538E datasheet cd74hc4538.pdf reveals that the only pins that are different in function between these two chips are pins 1, 14, and 15.

The main difference is that on the CD74HC4538E the RC gets it's ground from the CD74HC4538E directly, not from an external circuit board supplied ground. Also, the MC14538BCP has pin 15 connected to VSS.

With that in mind we don't want to connect pins 1 or 15 to the circuit board.

Instructions for fitting the CD74HC4538E to the MPP circuit board:

1) For both pins 1 and 15 of the CD74HC4538E - bend the pins up so they are horizontal (instead of pointing down).
2) For both pins 1 and 15 of the CD74HC4538E - cut the small part of the pins (that normally plug into the socket) off so that only the fat part close to the chip remains.
3) Remove (carefully) C8 and C10, and the MC14538BCP (U6) from the MPP circuit board.
4) Solder C8 to the tops of (across) pins 1 and 2 of the CD74HC4538E - apply solder sparingly.
5) Solder C10 to the tops of (across) pins 14 and 15 of the CD74HC4538E - apply solder sparingly.
6) Plug the CD74HC4538E with the soldered on cap's into the U6 socket, verify your MPP timings per the build doc or your own specs.

Notes:

The calc for the time constant for the MC14538BCP is R*C=Seconds

The calc for the time constant for the CD74HC4538E is .7*R*C=Seconds

The time constant for the CD74HC4538E is therefore shorter using the same R and C.

To keep the time constants about the same - swap C8 from 220pF to 330pF and swap C10 from 10nF to 15nF - or better yet experiment with these values.

[aries1470]

Thanks KRinAZ, very much appreciated Easy to follow instructions. Personally, I will be using machined sockets, so if I need to update/ upgrade any of the IC's, it is easy to reply.
I have ordered two boards, one for the normal and one for mods, testing out different changes, one at a time. Rheostat and potentiometer is already being upgraded to be electronically controlled with pushbuttons and memory and keeping the settings saved ;-) I also have a few other things I would like to experiment with, but I will wait until I receive the boards, and get the stock version running.

As for power supply, already NOT going to be using the 78L05 and 79L05 but other circuits. For example, MCP1702-5002E/TO, but it needs to be modified although it does give up to 250 mA, instead of 100 mA, while the LT1121IZ gives upto 150 mA, with a 6v input, or down to 5.5V with less mA and you just flip it around, no small breadboard mounting needed.
I do have some other ideas up my sleeve, but I will wait first to receive the boards since I am still new to gold prospecting ciruits, I want to complete something before I get carried away modding.

[KRinAZ]

Thanks KRinAZ, very much appreciated Easy to follow instructions. Personally, I will be using machined sockets, so if I need to update/ upgrade any of the IC's, it is easy to reply.
I have ordered two boards, one for the normal and one for mods, testing out different changes, one at a time. Rheostat and potentiometer is already being upgraded to be electronically controlled with pushbuttons and memory and keeping the settings saved ;-) I also have a few other things I would like to experiment with, but I will wait until I receive the boards, and get the stock version running.

As for power supply, already NOT going to be using the 78L05 and 79L05 but other circuits. For example, MCP1702-5002E/TO, but it needs to be modified although it does give up to 250 mA, instead of 100 mA, while the LT1121IZ gives upto 150 mA, with a 6v input, or down to 5.5V with less mA and you just flip it around, no small breadboard mounting needed.
I do have some other ideas up my sleeve, but I will wait first to receive the boards since I am still new to gold prospecting ciruits, I want to complete something before I get carried away modding.

thx & good luck with those mods!