SIMPLE, cheap latching circuit - one transistor?
xanatos
Posts: 1,120
Feeling somewhat dumb at the moment. I need a VERY simple latching circuit that will simply be on until a signal hits it (can be either + or -, I have no preference) and the circuit will remain off until I reset it.I have been playing with SCRs and stuff but apparently my current draw isn't sufficient to get them to stay in saturation.
So I went to a S/RS Flip Flop arrangement using an ANCIENT 7400 chip and that works well... as soon as a signal hits the input of one of the gates it flips and stays put until I put the signal to the Reset input and it goes back, no matter how many times I wiggle the input. (Note - the 7400 chip I used I have had since I was a kid - literally got that in the 1970s, and it's still working. Love those old 7400s!)
I remain convinced however that there should be a way to do this with a single transistor that feeds back onto itself, but I can't quite get it to work.
So I'm either looking for that magical single-transistor circuit, or a more up-to-date and perhaps more efficient circuit than the 7400s wired as S/RS Flip Flops. I need 5 of these circuits, that would be 3-7400 chips. Is there anything like a "hex S/RS Flip Flop" on a single chip? :-)
Thanks for the ideas, should any come forth. If worse comes to worse, I can live with the 7400 arrangement, but my gut is telling me there has to be something better.
Dave
Comments
If the SCR fails to latch, the problem may be with the load and not the SCR. A certain minimum amount of load current is required to hold the SCR latched in the "on" state. This minimum current level is called the holding current. A load with too great a resistance value may not draw enough current to keep an SCR latched when gate current ceases, thus giving the false impression of a bad (unlatchable) SCR in the test circuit. Holding current values for different SCRs should be available from the manufacturers. Typical holding current values range from 1 milliamp to 50 milliamps or more for larger units.
Dave
You can use a sweet little latching relay like http://www.goldmine-elec-products.com/prodinfo.asp?number=G2011 which only requires a brief pulse to change states. Or you can wire up any DPDT relay into a latching configuration, latching the coil connections through one channel using the NO contact. You reset by removing power briefly (NC switch typicallly) to open the relay contacts.
-Phil
-Phil
At first I thought 5v latching relays were a bit of a joke, I liked the bigger more powerful ones for home automation. But these tiny ones do offer a way to retain state and conserve power in battery operated devices.
With a 1/2 amp at 125VAC, these are a great way for a microcontroller backup battery to turn on and off a wallwart when a cyclical task needs power. When power one the battery can recharge.
With a CMOS non-inverting buffer, the output feeds back to the input and it holds its last state. But forcing the input, it changes state. It can be any non-inverting buffer, but the NC7SZ126 is a single unit in an SOT package. So, almost one transistor footprint.
A PUT (programmable unijunction transistor) has better controlled parameters, and can operate at considerably lower current. The valley current in the following circuit is on the order of 200µA, the peak point current around 1µA. A BJT or mosfet can be added to drive a heavier load.
I'm a big fan of PUTs. Wish they were available in SOT23. Alas, it is a dinosaur and only seems to survive in TO92.
Duane J
Duane J
Anyway. Now that I'm here, here is my solution.
Use a 2N7000 N-Channel MOSFET.
Drive a high efficiency LED from it's drain.
The light from that LED shines on a second similar LED connected to the gate. This second LED will generate a voltage, or allow current to pass when illuminated. LEDs are photo detectors as well as photo generators.
With a suitable LED and resistor (or two) on the gate we have:
When the FET is off and no input is connected no light shines and the gate is held low. Thus holding LED off.
When the gate is driven high, overcoming the LED/resistor biasing. The FET turns on, the LED shines, the receiving LED gets power and can hold the gate up when the input is disconnected.
Drive the gate low again and we reverse all that back to the starting point. Off.
Now, the switch on voltage of a 2N7000 is a bit over 2 volts so this may require multiple receiving LEDs or careful biasing.
It does require that whatever drives this to go into a high impedance state between transitions. But we can do that from a Prop pin.
I have no idea if this can be made to work. But I have seen how MOSFETS can be latched on and off just by the charge stored on the gate and nothing connected. So there should be some chance.
This calls for some experimenting...
Is it time to re-open the case for LED touch-sensing?
Is the jet on a treadmill? What color is the bear? Was an icicle in fact the murder weapon? How many were going to St. Ives?
The challenge is to build a latching circuit with only one transistor. Presumably with the minimum of other components.
Try to concentrate
If you can make a line following bot with only one motor this must be a walk in the park....
1) momentarily break the external anode/cathode circuit or
2) momentarily short the AK terminals together (bypass the SCR)
A normal SCR setup.
But how do you do 1) and 2) ?
Sounds like it needs some other transistors or whatever switches.
Maybe I'm wrong but by interpretation of the problem is: One wire in. And some how flip the latch on and off with that. And only one transistor.
* 74LVC1G175 - D-FF with /MR
* xx2G00 or xx2G02 wired as cross-coupled gate S-R latch
* Dual complementary transistor PNP&NPN, wired as PUT
I don't seriously think a one transistor latch is a good idea. But it's a challenge right?
None of the solutions strictly meet the 'one transistor' dictate, so it is less of a challenge, more plain bad semantics.
OK, with some LEDs.
And, OK it may never be workable.
Grief, do I actually have to build that now?
If you want to take that path, you can use a new Si8752 Isolated Gate Driver in an identical mode circuit.
Expensive? LEDs in my junk box?
I have no idea what an Si8752 is. But I'm sure it's as much work to get as getting the FET I wanted.
I just had a quick google and boy are there so many circuits out there that would never work and some while trying to implement positive feedback miss the fundamentals of bjts such as Vbe drop etc. One circuit even has the base wired to the emitter thinking that would work
Would storing a charge on a MOSFET gate count? If the cap was good enough and leakage was very low then it would work like an EPROM memory cell. Eventually all those EEPROM/EPROM/Flash memory chips out there will "de-latch" as they lose their gate charge. But in the meantime they work.
https://pdfs.semanticscholar.org/db1b/ff1537e3587862f23a1eabb1697161d84bcf.pdf
They describe a "negative resistance bistable circuit". Its composed of a single transistor and some diodes and resistors.
The UJT however is not a bad choice for such a thing. There are also devices called silicon controlled rectifiers. (SCR) When turned on they stay on until you remove the power to them. The ac equivallent turns off when the signal crosses 0.
I'm struck by how all the circuits in there are encrusted with diodes, capacitors and voltage sources.
I get the impression that in the modern world it's cheaper and faster to just use a bunch of transistors for such latches and flip flops.