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DIP vs SOIC 74HC132 behaviour

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earckens

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In the circuit attached (1st picture) I use succesfully a 74HC132 DIP package CMOS NAND Schmitt Trigger osillator (about 2.4kHz). I designed a pcb (at the rear of the 2nd picture showing both the protoype pcb and the production pcb) where the same IC is used in SOIC footprint. However I could not get the oscillator to function so I used a SOIC adapter board to use an SOIC in my protoype board (3d and 4th picture). Still the same, no oscillation: pin 4=H, pin 5 = pin 6 = H. I had ordered 2 batches of these SOIC's, so I tried one from Motorola, and only then would it work.
Conclusion: it seems as if SOIC's are more prone to batch failure.
Question: is that correct?
 

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jpanhalt

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Were all of the IC's from Motorola or just the one that worked?
I doubt "batch failure" per se is the culprit. More likely a counterfeit part (if not from a known distributor) or things like distributed capacitance and inductance that allowed some parts to work and not others in that circuit. Getting an oscillator to start can be as simply as touching one of the pins.
 

audioguru

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Hey, that is the Plants Watering Watcher project that I fixed. I made about 15 of them with American Name-Brand ICs and they all work perfectly.
Your NAND is defective if both inputs are high and the output is not low.
 

earckens

Member
Hey, that is the Plants Watering Watcher project that I fixed. I made about 15 of them with American Name-Brand ICs and they all work perfectly.
Your NAND is defective if both inputs are high and the output is not low.
Hi audioguru, indeed and I had posted a question to you on some other forum that you did answer a few months ago. Great circuit!! I use it with a controller to pulse once an hour for a few seconds and then transmit the value by radio to an ESP8266 master. I made an irrigation control system with it.
Were all of the IC's from Motorola or just the one that worked?
I doubt "batch failure" per se is the culprit. More likely a counterfeit part (if not from a known distributor) or things like distributed capacitance and inductance that allowed some parts to work and not others in that circuit. Getting an oscillator to start can be as simply as touching one of the pins.
The Motorola IC worked straight away, the other 3 (defectives) are from NXP.
But another culprit: 1206 footprint C1 (1nF): I replaced that with a discrete 1nF ceramic cap, and all works now.

I measured the 1206 package cap and it shows 1nF (Fluke 289), but when inserted in the circuit it does not charge: 5V remains over R3 (470k). Strange..
 

earckens

Member
Wrong diagnosis, I found a via shorted against pin 5.

But now pins 4&5 are H, pin 6 = L, yet C1 does not discharge over R1. I replaced R1 with a discretionary resistor, C1 again with a discretionary ceramic, same result. No short with Vcc measured. I know the schematic works, but what could be wrong here?
 
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