CD4538 schematic review (very much a noob) - Page 1
Forum section: electronics / beginners

author: Jays100
author_rank: Contributor
timestamp: February 15, 2025, 08:08:32 pm
content: I’m trying to design a timing circuit using CD4538 dual mono stable vibrator chips.The design intent is to from a hall trigger (OH137) into the first CD4538, one side turns on a transistor for 10ms (TIP35C). The same initial trigger starts the 4538 other side timer for 4.7ms which acts as a delay for a second 4538 trigger.The second 4538 outputs a 3.3ms signal to another external device, an automotive ignition smart coil.I’m an old school mechanical engineer, retired. Discrete electronics were always handled by the EE group so I’m not too smart about how electrons work.I’ve reviewed the various data sheets, truth table and “think” I have things properly connected, but I’d appreciate a review and correction of any and all of my mistakes.Here is a photo of my schematic:

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2025, 09:13:28 pm
content: Seems overly complicated to me. If you just want three different pulses, 1-1/2 4538s would be enough.Why use three 4538s (= 6 MMVs)?

author: Jays100
author_rank: Contributor
timestamp: February 15, 2025, 09:16:15 pm
content: Thanks for reviewing!I need two identical signals, one side separate from the other.

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2025, 09:55:00 pm
content: OK, so you're having two identical circuits that need to operate independently from each other? Is that right?In that case, just draw one side, much easier to read and analyze.

author: Jays100
author_rank: Contributor
timestamp: February 15, 2025, 09:59:36 pm
content: Thank you. I had thought about simplifying but decided a whole approach was best. Will do that shortly!

author: Jays100
author_rank: Contributor
timestamp: February 15, 2025, 11:09:26 pm
content: Here is one side. I think Ive identified an error. The 2nd IC triggers on the falling edge after 4.7ms, not the rising edge as shown.

author: ledtester
author_rank: Super Contributor
timestamp: February 16, 2025, 04:57:25 pm
content: Are you using 6V for the supply voltage? The CMOS version of the 4538 doesn't have that much output drive at that supply voltage (like around 1 mA) and I wonder if that will be enough to drive the TIP35C in your application.Also, you may need some power-on-reset circuitry to ensure the 4538's don't glitch their outputs when powered up.

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 05:10:36 pm
content: From what I could see, I can’t use 12v for the whole circuit. Still, I need a switch to turn on 12v/3A for 10ms as one of the use needs. Is there a better way?

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 16, 2025, 05:37:05 pm
content: I see that you are up-front accepting your noob status and are humbly requesting assistance. This is a good attitude! You'll receive a lot of help.This is my two yen:CD4xxx series logic are perfectly capable to operate up to 15VDC. The CD4xxxBcan operate up to 18VDC. Thus you can safely feed them 12 VDC.With respect to your schematic: it is actually quite understandable, but I would strongly encourage you to use the proper transistor symbols. I have attached the TIP35C pinout for your reference..

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 05:46:20 pm
content: Thank you, I will amend and repost, I am indeed a noob in the EE world.As to my thoughts on a schematic error, I think at that point is okay after all.My dilemma is the smart coil only accepts a 0-5v signal, is TTL logic. So, where should the 7806 (or 7805) be best placed in the circuit?

author: ledtester
author_rank: Super Contributor
timestamp: February 16, 2025, 06:44:29 pm
content: Quote from: Jays100 on February 16, 2025, 05:10:36 pmI would like at using a MOSFET to do the switching.There are a couple of issues with the TIP35C... 1) the base current required to switch 3A and 2) the V_CE at a collector current of 3A which the datasheet suggests will be at least a couple of volts -- i.e. your load won't get the full 12V but only 10 or less.For a MOSFET I'd look at this one:https://www.adafruit.com/product/355Datasheet:https://cdn-shop.adafruit.com/datasheets/irlb8721pbf.pdfAnother popular choice is the IRLZ44N:https://www.mouser.com/catalog/specsheets/international%20rectifier_irlz44n.pdf

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 07:07:36 pm
content: Thank you, all, for your input.Will make the schematic change(s) and incorporate a better MOSFET. As it happens, it’s not a huge problem if 10v. 12 is best, but 10 still works.Apologies if this was addressed and I missed it. If my working voltage is 12 - correct for the first part of this puzzle - then how do I get to 5v for the second part of the puzzle? Can I output from the 4538 pin 6 directly through a 7806?

author: ledtester
author_rank: Super Contributor
timestamp: February 16, 2025, 08:05:52 pm
content: Quote from: Jays100 on February 16, 2025, 07:07:36 pmIf you are trying to use the 7806 as a voltage level translator -- no, that won't work.Do you have a datasheet or part number for the automotive ignition smart coil? There are a lot of ways to level shift a signal -- e.g. some of them are discussed here:https://electronics.stackexchange.com/questions/82104/single-transistor-level-up-shifterbut your situation may have special requirements.

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 08:21:34 pm
content: The coil I want to use is a GM D510C, with onboard igniter.From trigger, Pin D gets 12v for 10ms. After a delay of 5ms, pin C receives a +5v signal for 3.3ms.Spark occurs.

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 08:24:33 pm
content: In my ignorant logic, I had thought that simple timers cascaded would give the timing and delay functions. What I’m realizing is matching components is definitely an art I lack.Again, thanks to all for herding me in the right direction.

author: Jays100
author_rank: Contributor
timestamp: February 16, 2025, 11:48:05 pm
content: Would a simple voltage divider work for the 3.3ms command?

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2025, 01:22:23 am
content: Quote from: Jays100 on February 16, 2025, 11:48:05 pmPossibly - it depends on the nature of the trigger input.This ignition coil:https://www.amazon.com/IGN1A-High-Output-Smart-Ignition/dp/B07JJDNK18says the trigger is "TTL". This one also says "TTL" on the label:https://hyperaktive.com/product/hyperaktive-smart-ignition-coil/This article:https://www.classe.cornell.edu/~ib38/teaching/p360/lectures/wk09/l26/EE2301Exp3F10.pdfhas this to say about CMOS driving TTL inputs:Quotebut this is also assuming no level shifting is involved. Is the ignition coil's "TTL" input standard TTL or low-power TTL?The 74C901 doesn't seem to be readily available anymore.Another standard IC to level-shift and convert CMOS signals to TTL is the CD4050B:https://www.ti.com/lit/ds/symlink/cd4049ub.pdfbut I don't know if there are other issues to worry about. In this case the CD4050 would be powered from a 5V supply and you just feed the higher voltage CMOS signal directly into the CD4050's input.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 17, 2025, 11:37:52 am
content: Its taken a while to unravel your wiring diagram. Wiring diagrams are not really the same thing as schematics- they use symbols not ic package footprints.There are plenty of free easy to use CAD packages with "schematic capture" Well worth using.If this is going to run on automotive battery power its worth applying protection components. Automotive battery volts have plenty of transients that will happily over volt the regulator and the CMOS logic. As a minimum a series diode and a transient absorber diode (aka TVS diode) should be used prior to the regulator, eg SMBJ15A. We're talking a few cents here.You need to drive that TIP35 into saturation.A TIP35 has an current gain of about 50. That means for every amp of collector current you need at least 20mA of base drive.A 4538 can manage about 5mA on a good day. There are better options (not Darlingtons guys!).A MOSFET, eg an IRF540 (100V 33A) would be a lot easier to drive from the 4538. Use a 10-100 ohm resistor between Q and the gate. It helps reduce ringing.A snubber diode is needed to stop the coil back emf from the coil frying the MOSFET. Better still, use an Automotive IGBT eg an RGPR20NL43HRTL which is built to take the back emf punshiment with no futher protective parts required.I see you have used individual power-on clear RC circuits. You only need to use one RC and tie all the /CLR (3,13) pins together. Well done for using a propper monostable and not 555s. You migh consider using the non-retiggerable configuration to lock out spurious triggers during the timing period(s).

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2025, 11:46:19 am
content: Quote from: Terry Bites on February 17, 2025, 11:37:52 amShouldn't that 1N4007 across the MOSFET be a TVS, zener or RC snubber network? That 100V-rated MOSFET is going to avalanche over long before the 1kV-rated diode.

author: Jays100
author_rank: Contributor
timestamp: February 17, 2025, 01:11:47 pm
content: Good morning Mr Hunt - Today’s mission, (should you decide to accept) is to find and utilize schematic software, incorporate collective wisdom, redraw and repost. Plan on nothing fried, baked or smoked.Good thing I like coffee!

author: Jays100
author_rank: Contributor
timestamp: February 17, 2025, 10:23:32 pm
content: Here’s where I am, starting with a proper schematic - using Kicad, well, learning anyway.Addressed reset pins 3/13Addressed coil charge transistor to PNPN type IRGB14C40L IGBTChanged to 12v overallAddressed pin 6 output through a 2N2222 and 7805Not addressed is any filtering, still somewhat unclear what’s needed.Not addressed is deciphering the truth table for non-retrigger ops. Question: the hall switch is a negative edge trigger to pin 5. Is pin 4 then connected to pin 6 and not gnd? But, pin 6 output is needed turn on the IGBT transistor, how does that work?

author: Jays100
author_rank: Contributor
timestamp: February 17, 2025, 10:29:10 pm
content: Schematic

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2025, 10:41:55 pm
content: Quote from: ledtester on February 16, 2025, 04:57:25 pmIt seems that the 74HC/HCT versions of the IC have a power-on reset circuit built-in. From the OnSemi/Motorola datasheet for the MC74HC4538A (page 11):https://www.onsemi.com/download/data-sheet/pdf/mc74hc4538a-d.pdfQuoteI then noticed that even though it doesn't talk about it the TI datasheet for the CD74HC4538 seems to have POR circuitry shown in its schematic (page 3):https://www.ti.com/lit/ds/symlink/cd54hc4538.pdf(see attachment)However, none of the old datasheets for the 4000-series ICs seem to mention POR or show it in their logic diagrams, e.g.National:https://dmohankumar.wordpress.com/wp-content/uploads/2011/05/cd4538.pdfFairchild: see attachmentSo, for the 4000-series POR is likely necessary but not for HC/HCT.

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2025, 11:09:57 pm
content: Quote from: Jays100 on February 17, 2025, 10:29:10 pmUnused inputs of U1B and U2A should be tied to a rail (either Vdd or Vss) - i.e. pin 11 or U1B and pin 5 of U2A.Also, you should do the same for all of the input pins of U2B - the other multivibrator you aren't using.Also, it seems you want to perform high-side switching with Q1. Are you actually going to use a IRG4PF50W? It looks like you might have the collector/emitter of the IGBT backwards.For Q2 I'd use a P-channel MOSFET instead of an NPN.Edit: Is Q2 supposed to be your TTL signal to the smart ignition coil?

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2025, 11:25:13 pm
content: Quote from: Jays100 on February 17, 2025, 10:29:10 pmExcellent for a first-timer!

author: Jays100
author_rank: Contributor
timestamp: February 17, 2025, 11:55:07 pm
content: I may have mentioned I’m a total noob at this…Yes, Q2 is my 5v/TTL signal intentYes, quite likely my connects are backwards, will recheck!

author: Jays100
author_rank: Contributor
timestamp: February 18, 2025, 03:09:31 am
content: The plan is to use IRGB14C40L, Kicad do didn’t have that in the library so used something else. Learning the edit functions…Yes, 5v switching through Q2, will that work?

author: ledtester
author_rank: Super Contributor
timestamp: February 18, 2025, 07:02:08 am
content: Quote from: Jays100 on February 18, 2025, 03:09:31 amThe IRGB14C40L is described as an "ignition IGBT".Datasheet:https://www.digikey.com/en/htmldatasheets/production/74033/0/0/1/92-0235I would have a look at this video:How to Use Ignition IGBTs and Driver ICs -- FairchildSemihttps://youtu.be/raSpY6UvpWoand note the following:1) They use a special IGBT driver IC between the output of the function generator and the IGBT.2) See the diagram at the 1 minute mark... the IGBT is between the coil and ground -- not between +12 and the coil. You'll see the same arrangement on page 7 of the IRGB14C40L's datasheet.

author: xvr
author_rank: Frequent Contributor
timestamp: February 18, 2025, 10:52:18 am
content: There is no clock signal to U1BTime setting circuit shorted to ground for U1A/BSmall note to improve readablitiry:Do now draw Ground and Power wires as wires - use Ground/Power symbols. Ground/Power Wires will clobber schematics.

author: jfiresto
author_rank: Frequent Contributor
timestamp: February 18, 2025, 11:11:14 am
content: Quote from: ledtester on February 17, 2025, 10:41:55 pmThe TI/Harris/RCA CD14538B appears to have a power on reset (see attachments).

author: Jays100
author_rank: Contributor
timestamp: February 18, 2025, 01:02:36 pm
content: I think I need to back up. The intended smart coil has an on board Igbt. I should have asked this earlier, why is another needed?

author: Jays100
author_rank: Contributor
timestamp: February 18, 2025, 01:09:19 pm
content: Further backing up…the coil saturates primary within 6ish ms via pin D. If pin C is not energized, the coil will auto fire. This is why the 10ms window is opened, the 4.7ms delay saturates the primary and the field collapses for 3.3ms when the whole thing is timed out and the window closes.Am I going about this wrong?

author: Jays100
author_rank: Contributor
timestamp: February 18, 2025, 05:26:46 pm
content: new schematicchanged igbt to IRF9540 P channel Mosfet for high side switching (Thank you!)verified that Q2 as N channel is okay with Gate 12v for Source 5v switchingcleaned up Supply and Gnd connectionsfixed hall switch connection, oops! When the 10ms charge window opens, also needs to start 4.7ms delay.rail connection for unused pins

author: jfiresto
author_rank: Frequent Contributor
timestamp: February 20, 2025, 11:20:05 am
content: Shouldn't the ground go to the CD4538's pins 1 and 15?Also, you can use much higher value timing resistors and correspondingly smaller, cheaper, and/or more reliable timing capacitors.This is from the last (non-automotive) CD(1)4538 circuit I designed:

author: Jays100
author_rank: Contributor
timestamp: February 20, 2025, 11:59:21 am
content: Thank you, yes, an error on pins 1 & 2 gnd.Regarding better RC reliability, the literature didn’t mention so I used simple math. If 10M resistor /.1uf cap were used, is that a more reliable combo?I’m using this as a one-shot, the hall switch goes low at trigger on U1A pin 5. Pins 3, 13 are resets.