EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: DaAwesomeP on March 13, 2016, 06:34:06 pm
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I'm making a boards to play around with digital logic. I need most of the logic types. The whole thing will be 6v (4 AA batteries). The idea is that I'll have some sort of connector (probably banana) that can go between the inputs and outputs as well as LEDs. All of the inputs and outputs need to be 6v (or close enough to that). I'm basically making "breakout boards" for these chips.
I've gone with the 74HC series since it seems pretty widely used and available (I found chips from both ON and NXP). Fairechild has the 7S and 7SU series. These are the parts that I've selected so far:
- AND - 74HC1G08
- OR - 74HC1G32
- NAND - 74HC1G00
- NOR - 74HC1G02
- XOR - 74HC1G86 I might skip this one if I have trouble getting it. It seems that only NXP makes it.
- Inverter - 74HC1GU04 or 74HC1GU14? What's the difference? The 14 lists the logic input voltage in the datasheet, while the 04 doesn't.
Digikey search with all of these chips (https://www.digikey.com/product-search/en/integrated-circuits-ics/logic-gates-and-inverters/2556317?FV=1c0002%2C1000001%2C4500022%2Ca6c0001%2Ca6c0002%2Ca6c002d%2Cffec006e%2Cfff40027%2Cfff8019d&mnonly=0&newproducts=0&ColumnSort=-1000009&page=1&stock=1&pbfree=1&rohs=1&k=&quantity=0&ptm=0&fid=0&pageSize=500)
I'm confused with the inverter. I can't tell the difference between the 04 and 14 except that the 14 seems to have different input voltages for the inputs, but the 04 doesn't specify that at all.
If there's a better way to do this (different chips, voltage, or anything else), please tell me!
Thanks,
DaAwesomeP
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Why not go with standard 5V logic? If you are playing around on a patch board you really don't want anything faster than 74HC or 74LS. However real TTL logic is a power hog so you may want to avoid LS. I'd use a boost converter to go from 2xAA to 5V with very high efficiency. Alternatively use 4000 series CMOS direct from the battery (as 4000 series CMOS has a very wide supply voltage range and would be quite happy on 4xAA, with no problems over the full discharge curve), but that makes it much harder to interface to MCUs e.g. you may want to add an Arduino Nano v3 for generating logic pulse sequences, clock signals or for use as a crude logic analyser.
For patching you certainly don't need anything as big and clumsy as banana plugs. Instead, I recommend using standard breadboard jumper cables with 0.5mm dia turned pin ends, and compatible female 0.1" pitch headers on the PCB.
You wont get very far with just simple logic gates, I'd add counters, decoders, flipflops and shidft registers to my selection. You need to make a decision whether to make dedicated logic modules with the inputs and outputs arranged according to the logic symbol, with series resistors for each output for short circuit protection, indicator LEDs and facilities to jumper unused inputs high or low, or to simply provide uncommitted 14 or 16 pin sockets, with only power and decoupling prewired, (or possibly a combo socket with a jumper for Vss pin selection) that you can simply stick any power pin compatible chip in, with its pinout expanded to 0.2" pitch next to it with space to slip in a card printed with its logic diagram and pin names.
N.B. decoupling is *ESSENTIAL*. I recommend 0.1uF ceramic across Vdd and Vss of each logic chip + 100uF electrolytic where the wires from the battery holder reach the board.
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- Inverter - 74HC1GU04 or 74HC1GU14? What's the difference? The 14 lists the logic input voltage in the datasheet, while the 04 doesn't.
I'm confused with the inverter. I can't tell the difference between the 04 and 14 except that the 14 seems to have different input voltages for the inputs, but the 04 doesn't specify that at all.
I could not find the 74HC1GU14 at www.datasheetarchive.com (http://www.datasheetarchive.com) but the normal size 74HC14 is a bunch of "Schmitt Trigger" inverters. The Schmitt Trigger has a snap action when the input voltage reaches the switching threshold voltages.
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Thanks for the feedback!
Why not go with standard 5V logic? If you are playing around on a patch board you really don't want anything faster than 74HC or 74LS. However real TTL logic is a power hog so you may want to avoid LS. I'd use a boost converter to go from 2xAA to 5V with very high efficiency. Alternatively use 4000 series CMOS direct from the battery, but that makes it much harder to interface to MCUs e.g. you may want to add an Arduino Nano v3 for generating logic pulse sequences, clock signals or for use as a crude logic analyser.
For patching you certainly don't need anything as big and clumsy as banana plugs. Instead, I recommend using standard breadboard jumper cables with 0.5mm dia turned pin ends, and compatible female 0.1" pitch headers on the PCB.
You wont get very far with just simple logic gates, I'd add counters, decoders, flipflops and shidft registers to my selection. You need to make a decision whether to make dedicated logic modules with the inputs and outputs arranged according to the logic symbol, with series resistors for each output for short circuit protection, indicator LEDs and facilities to jumper unused inputs high or low, or to simply provide uncommitted 14 or 16 pin sockets, with only power and decoupling prewired, (or possibly a combo socket with a jumper for Vss pin selection) that you can simply stick any power pin compatible chip in, with its pinout expanded to 0.2" pitch next to it with space to slip in a card printed with its logic diagram and pin names.
N.B. decoupling is *ESSENTIAL*. I recommend 0.1uF ceramic across Vdd and Vss of each logic chip + 100uF electrolytic where the wires from the battery holder reach the board.
I was thinking about 5v for a bit, but I don't really see a need for it (yet). What's nice about these chips is that I could switch to 5v if I add in an MCU or other stuff later. I
was thinking about using regular pins (breadboard style), but I opted for banana plugs for several reasons: 1) I might give this to kids, and they don't bend, break that easily, poke your fingers, and are easy to connect/disconnect but don't slip out easily, 2) they're stackable, and 3) they're still relatively cheap if I order them in bulk.
I want this to be pretty basic, so anything that needs a clock would greatly increase the complexity of the system.
I'm going to have a whole bunch of power and ground connectors to go to the inputs of the logic chips and power the other components. I'm thinking about hidng the VCC of the logic chips (not making connectors for them). The board will have other stuff on it (buttons, switches, potentiometers, LEDs) as well. They circuit board will be hidden in the enclosure, so I will have symbols and labels on the outside.
I had not thought about decoupling. I'll add that into my plans.
I could not find the 74HC1GU14 at www.datasheetarchive.com (http://www.datasheetarchive.com) but the normal size 74HC14 is a bunch of "Schmitt Trigger" inverters. The Schmitt Trigger has a snap action when the input voltage reaches the switching threshold voltages.
Here are the Datasheets: 74HC1G04 (http://www.nxp.com/documents/data_sheet/74HC_HCT1G04.pdf) and 74HC1G14 (http://www.nxp.com/documents/data_sheet/74HC_HCT1G14.pdf). What is the difference between the Schmitt Trigger inverter and a regular inverter?
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For the 74hc series 6 V is typically (except analog switches) the absolute maximum rating.
New AA Alkaline cells can have a little more than 1.5 V, more like 1.6 V so this is too much.
But 74HC works perfectly well with 2.7 V, so the logical choice would be 2 or 3 cells, not 4.
For first test I would prefer DIP versions - no need for PCBs.
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You'll need to worry about input ESD protection and output short-circuit protection then. Kid-proofing CMOS logic without a lot of extra parts is not so simple and the costs of doing so make it difficult to include enough gates to make it capable if anything interesting or fun so it becomes purely a limited educational tool, used for one chapter of a science & technology textbook.
OTOH breadboard-compatiible connectors + a breadboarding area + socketed logic chips is IMHO the way to go for any kids old enough not to chew the parts. The pre-assembled flexible breadboard jumper cables have rounded ends so its quite difficult to prick your finger on one. If a logic chip is damaged, its cheap and easy to replace. The only significant hazard is treading bare-foot on a discarded chip that's landed pins up, and that in itself is a learning experience.
@Kleinstein: 3 cells, not 2. Otherwise the battery life will be lousy. With 3 cells it can run right down to 0.9V/cell which is nearly dead, but with only two, it browns out at 1.35V/cell with 80% of the battery capacity remaining. :(
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Thanks for the feedback!
Here are the Datasheets: 74HC1G04 (http://www.nxp.com/documents/data_sheet/74HC_HCT1G04.pdf) and 74HC1G14 (http://www.nxp.com/documents/data_sheet/74HC_HCT1G14.pdf). What is the difference between the Schmitt Trigger inverter and a regular inverter?
With a Schmitt trigger inverter, the input threshold voltage for logic 0 to 1 (at input) transition is higher than the threshold for 1 to 0 transition.
Therefore, with a slowly-changing input, there is a "snap action" to avoid ill effects near the threshold point. These threshold voltages should be specified on the data sheet, with tolerances.
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You'll need to worry about input ESD protection and output short-circuit protection then. Kid-proofing CMOS logic without a lot of extra parts is not so simple and the costs of doing so make it difficult to include enough gates to make it capable if anything interesting or fun so it becomes purely a limited educational tool, used for one chapter of a science & technology textbook.
Most 74HC parts seem quite resilient. Including protection input (>1 k?) and output resistors (~300 ?) might be a good idea anyway. If output LEDs are dedicated, output resistance can be made higher, by including dedicated drivers (small NPN) for each LED.
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If you want a wide supply voltage range you're better off with the 4000 series. They're actually quite useful as drivers and level shifters.
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I have dealt with this two ways before. As mentioned above going to CMOS 4000 series is a good option. If you must stay pure 74 series go with 74HC chips. 74HCT chips are designed to adhere to TTL standards and do not want any more than 5 volts anywhere. 74HC chips have generally a 6 volt tolerance and some more if you are daring, but i cannot recommend it for anything other than hobby use. If this is something for sale I would recommend one of two options i have used. First, use a little buck converter. They are super cheap and these chips will happily work down to 3 volts generally. feed your 6 volts from your batteries into a buck converter putting out 3.3 or 3.6 volts and you should be in good shape and have good battery life. Not as good as CMOS but acceptable. The other way is stick a diode in series with the supply to drop the voltage. A nice red LED as an ON indicator works well and drops a little over a volt. Again remember these chips work down pretty low in voltage so your life will be fine.
Those are the things that came to mind.
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I like the suggestion of using 3 batteries in series as the 74HC will also work with much lower voltages than 5V. If you want to make it rugged I'd put 470 Ohm resistors in series with the outputs and inputs. A small capacitor or even better a varistor at the inputs will improve ruggedness as well. You'd have to test this to make sure but it appears modern HC chips are resillient against applying reverse power. Otherwise a series diode in the power supply will do the trick. And don't forget the 100nf decoupling capacitor across the power pins. For fun you could put leds on the building bricks so kids can see the logic levels being applied to the brick. If you use chips with 4 gates in one package you can use the extra 3 gates for buffering the leds.
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Wow! Thank you all for the feedback!
I'm starting to think about just putting in a Pro Mini or Adafruit's Trinket Pro (both are $10 USD). That would give me the option for 4 AA or USB power as well as future expansion. I would use a diode and resistor on each of the input/output pins. However, would I need a pullup on all of the input pins?
I think that this would be enough protection and it's very simple, and I wouldn't have to order any special boards. In fact, anyone could replicate my work easily at home. No one will ever have to know that it isn't actually logic chips ;D
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For the 74hc series 6 V is typically (except analog switches) the absolute maximum rating.
New AA Alkaline cells can have a little more than 1.5 V, more like 1.6 V so this is too much.
The absolute maximum rating for 74HC is 7V so 1.6V per cell shouldn't do it any harm. If this is a concern, add a diode in series with the batteries, which will also provide protection against reverse polarity.
http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdf (http://www.nxp.com/documents/data_sheet/74HC_HCT04.pdf)