I’m having trouble with my quadruple gate AND chips (74HCT08) and my quadruple gate OR chips (74HCT32). The issue is as follows: when both inputs A and B of the AND/OR gate are low, output Y is high at 5v. When the criteria are met for Y to be high (for OR gate A or B is high, for AND gate A and B is high) the output pushes more current. This can be seen when an LED is placed between Y and ground, as when the criteria are met the LED becomes brighter. This is not good, I need the output to be completely low. I can find nothing in the datasheets about it, and I’m confident I have the correct pinouts for each and the correct supply voltage at 5V. These issues are persistent across all four individual gates on each chip I bought.
I’ve attached the datasheets I have been using.
Thanks!
Hi Alexander
Circuit diagram please.
When you say you placed a LED between Y output and ground you did include a current limiting resistor didn’t you???
I am not sure these logic ICs are meant to drive LEDs. You could have destroyed them. Not sure so please include an accurate circuit diagram of EXACTLY what you have.
Cheers Bob
Hey Bob,
Thanks for responding. Attached is my circuit diagram for one of the chips, the quad 2-input AND gates. For the 74HTC32 I just swap it in place as they share pinouts and voltage ratings. Is 220 ohm enough? It was the highest I had lying around. Thanks.
Hi Alexander
I am pretty sure these Ics are not meant to drive LEDs.
This copy from the data sheet leads me to that conclusion although to be honest I have never had the need to or tried to drive a LED
• Wide Supply Voltage Range from 4.5V to 5.5V
• Pin Compatible with Low Power Schottky (LSTTL)
• Inputs Are TTL Voltage Level Compatible
• Sinks or Sources 4mA at VCC = 4.5V
• CMOS Low Power Consumption
• Schmitt Trigger Action at All Inputs.
Note line 4. Although the sheet says different this 4mA is more like what is to be expected. With the 220Ω resistor you would be sourcing 10mA. If at any time I have needed to drive a LED I have always buffered with a voltage follower OP Amp or similar. Try replacing the LED/resistor combination with a DMM (digital voltmeter) and just measure the voltage.
If you have damaged one you probably have bricked the lot but there again they may be OK. Nevertheless I myself would be inclined to discard all of the ones you have tried to be on the safe side. They are not that expensive and could save a lot of heartache later.
Cheers Bob
Thanks Bob. I think you’re likely correct in that by testing with an LED first I may have bricked the lot permanently, as even with my DMM the output with low A & B sits at 3-4v and with high A & B sits at 5v. I will buy a couple more and perhaps in the final circuit use MOSFETs/transistors to control LEDs as I require them for output.
Thanks,
Alex
What is the intended usage for these devices? With a 220 ohm resistor and a LED you are testing close to the maximum rating and well above the recommended rating. It is much better to test them at the current draw you intend to use them at, which would typically be 4 to 8 mA for LSTTL.
The intended usage for these ICs was to construct a simple binary adder circuit with LEDs as a binary output, eventually a 7-segment display and subtraction functionality also. Clearly I didn’t pay as much attention to the datasheets before I ordered as I should have. Will have to check more closely when sourcing more or use MOSFETs/transistors to control LEDs as visual output.
Hi Alexander
You may not have bricked these ICs. I just had a look at the TI data sheet which is a bit more informative than the one you linked. Came across this paragraph.
Quote.
When using multiple-input and multiple-channel logic devices inputs must not ever be left floating. In many cases, functions or parts of functions of digital logic devices are unused; for example, when only two inputs of a triple-input AND gate are used or only 3 of the 4 buffer gates are used. Such unused input pins must not be left unconnected because the undefined voltages at the outside connections result in undefined operational states. All unused inputs of digital logic devices must be connected to a logic high or logic low voltage, as defined by the input voltage specifications, to prevent them from floating. The logic level that must be applied to any particular unused input depends on the function of the device. Generally, the inputs are tied to GND or VCC, whichever makes more sense for the logic function or is more convenient.
End quote
It seems that all inputs must be logic HIGH or logic LOW and must not be floating. If left floating the output could be anything apparently.
So your input switches you used to test should switch the inputs HIGH or LOW. You seem to have left them float if not HIGH. Try again with double throw switches and have the input switched to a definite state instead of float. This will probably work.
If you are going to use LEDs as indicators you should buffer them and NOT drive them direct from a logic IC.
Cheers Bob
PS. Test with your DMM, not an unbuffered LED.
In lieu of a DMM you could use a Logic Probe.
Jaycar only have such in kit form
Cat no KD6100
Whether you get one or not have a look and down load the Manual (in Downloads section). Ir describes these devices and seems to be good reading for what you are doing and I recommend you peruse this.
Logic probes are not uncommon and I daresay there are other suppliers who have these devices in complete form.
Cheers Bob
You’re 100% correct! By connecting to GND rather than leaving it floating the output is successfully pulled low on both chips. That data sheet would have fallen on deaf ears with all the technical terms in it, so your explanation was really excellent and this knowledge will serve me very well in the future. I will definitely have a look over that Jaycar manual you mentioned also. Thank you so much!
Discrete MOSFETs or transistors are not necessary. There are buffer or driver ICs that are designed for higher current devices. If you go here: Buffers, Transceivers & Line Drivers | element14 Australia
you can apply appropriate filters until you get a suitable range of ICs to choose from.
Bob and Murray have given awesome responses here, the only thing I’d like to add is that some logic gates I’ve run into have open-collector / open-drain outputs, which is handy when you want to combine 2 outputs without a third gate, but means you need pullups, as these sorts of outputs can only connect/disconnect a path to ground, and not drive a pin hugh.
Keen to hear about your project once you’ve got some fresh gates in there and buffers/ lower current limits!
Hi James
The open collector types that come immediately to mind are comparators LM393/339 which can be ORed simply by connecting several outputs together. Some devices even have both collector and emitter bought out to external pins, very versatile
Most Opto couplers are open collector and even some of these make the emitter available.
Most Darlington arrays are open collector and are generally used as small relay drivers etc.
But you are right, there are a lot of these devices out there
Don’t know the current convention but it used to be :— If there is a small section of arc drawn inside the angle formed by the output of an Op amp symbol that symbolised open collector / drain output.
Cheers Bob
