The Real Single Transistor Latch

Sometimes you need to plug things in backwards.

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The 1-transistor Flip-Flop project (and similar) on this site inspired me to try my own version of this. It is based off the negative resistance reverse breakdown behavior of a BJT, such as used in those backwards flasher circuits. I removed the capacitor, and tried biasing things differently to try to find a latched state, similar to negative resistance neon bulb latches.

The power supply voltage is critical, and must be adjusted per transistor. This is because reverse conditions of transistors are uncommon, so I don't think it's controlled very well in manufacturing.

Please try to duplicate on your own workbenches. I have tried it with two 2n4401s, just to make sure I didn't accidentally 'cook' one of them earlier into a different device with previous experiments. ( I swear I cooked a green LED to into a yellow on

I need to write some details here.  But for now, take a look at other's attempts to build single transistor latches:

In one of the comments, the reverse breakdown transistor flashed circuit was mentioned.  There is a writeup on hackaday about it:

Someone suggested using the negative resistance region as a latch, similar to neon tube latches.  I don't think anyone attempted it, otherwise we would have heard about it.  I tried to find a voltage where the transistor would either stay on or stay off, based on its previous state, and couldn't find it.  So I started experimented with different biasings.  Also added and removed capacitors to try to slow down the response of the circuit so I could watch what was happening.  Then I found it:  Add a resistor between the base and the collector.  It works.  But I'm not sure why.

  • Thanks Everyone!

    Mark Sherman05/12/2019 at 19:33 0 comments

    I didn't expect this to attract as much attention as it did.  I have a lot of reading to do on negative resistance devices.  I caught a cold in the last couple days, so I haven't had much time or energy to do any further experiments, but I do want to make measurements across the different junctions and try to characterize what is happening.

    I know it's not a very practical circuit, and probably won't be reliable, but I like strange, goofy things.  I found that there were multiple attempts by different companies to create 1-Transistor SRAM... everything from self refreshing DRAM that pretends to be SRAM, to SCR and Thyristor based designs, and they all kinda didn't work out as expected.

    I have found this so far:

    1. The off state isn't really off.  If I turn the lights out the LED glows very dimly.  The circuit just has a high power and low power state.
    2. The circuit works without the LED.  If remove the LED, and drop the supply a couple volts, a voltmeter will read two voltages for the two different latched states.  So there's no weird interaction with the diode; it is really truly a single active transistor.
    3. I left it on the off state overnight and it was still off.  I turned it on, and left it another day and it was still on.  I didn't watch it in between, but it seems very stable.
    4. I have not ruled out thermal effects.  Overnight the temperature in the house changed several degrees.  But, I have found if the supply voltage is just right, and I have it in the 'off' latch state, if I remove the Rbc resistor, it does not turn on immediately.  It takes about half a second to move from off to on.  If I momentarily short the base to ground, it turns off, and then float the base, there is another half second delay.  There are no capacitors, except for whatever capacitance the transistor has, plus maybe the small amount the breadboard has.  But it doesn't seem enough to account for the delay.
    5. If I put a capacitor in parallel with Rbc (say .1 uF), and measure its voltage in the on latched state, it wobbles quite a bit.  I've seen it go from .6 to .9 back to .6, over the course of a few seconds, but still keeping the LED on.  I have no idea what causes this random variation.  The power supply is a wall wart smoothed out with a LM317.  I would expect 120hz ripple if anything.
    6. I have duplicated both the latching behavior and time delay with a battery supply.

    I hope to revisit this sometime soon next week.  Maybe I'll try writing a paper for the hackaday journal.

  • 2n2222 sucess

    Mark Sherman05/08/2019 at 22:19 0 comments

    At work we have a little Arduino and electronics breadboard kit on a random table to play around with.  No one uses it for much, but it had a 9V battery and a 2n2222.  I wired up the circuit, and using a 100k resistor for Rbc, and 330 ohms for Rled, recreated the same circuit.  The battery read out about 9.2 volts on the multimeter.  I feel more confident in the circuit since the LM317 is no longer a factor (long-shot that the power supply had some interaction with the transistor), and that it works with at least a different transistor number, sourced from somewhere completely different.

    The next thing I need to try is removing the LED.  It is, after all, a diode, and as we've seen from other projects on this site (the DIODE clock!) diodes like to be 'active' devices from time to time.

  • video demonstration

    Mark Sherman05/08/2019 at 03:54 0 comments

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Ted Yapo wrote 05/08/2019 at 23:07 point

This just hit the front page, so be sure to check there for comments - readers may have some insight.

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Mark Sherman wrote 05/08/2019 at 19:41 point

I will try it without the LED, I just might need to reduce the supply voltage by whatever the LED voltage drop was.  I might have a flashlight bulb I can try too.. or I can just measure the current over the collector resistor.

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RoGeorge wrote 05/08/2019 at 11:48 point

Very interesting, nice finding!

Didn't tried to replicate it yet, but I intend to.  My bet is the latch effect is happening because of the interaction between the transistor and the LED.  Without the LED (or at least a diode as a second active device) most probably it won't latch, but this is just a speculation for now.  Anyways, I'm very curious to test your schematic.  Thank you for sharing.

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