Transistor latching circuit problem

Phil-S

Joined Dec 4, 2015
140
A CMOS logic circuit might save you a lot of grief.
One very old but good book on the subject (Author Marston title something like "CMOS logic" , or Lancaster similar) will probably provide a solution
There are plenty of examples around for using a push button to emulate a mechanical toggle switch.
 

Plamen

Joined Mar 29, 2015
98
Hi all
I've made a latching switch using a transistors but it's not working as it supposed, and i don't know why
When pressing the push button the led is lit but it's not latched since it turns off when depressing the switch
pls help why it's not latching the output voltage?
View attachment 103656
Petkan:
The circuit should work (and it does so in LTSpice). There is no need to add diodes or any parts..
What needs checking is the actual implementation. As correctly pointed Vbe voltages cannot exceed 0.8V.
Check that the transistor exhibiting higher Vbe is correct type and operational. It is easy to mistakenly swap NPN and PNP.
Check first initial condition when powered and no button pressed. Both transistors should be off (Vce around 5V for both), LED off (no voltage drop across its ballast resistor R2). Then take a resistor say 1k to 10k one end to GND and touch Q2-b. This should turn on the LED and prove Q2 works. The reconnect your test resistor to +5V rail and repeat the test with Q1-b. This should prove Q1 working. By the way either way you should see latching. For those uncomfortable without diodes note that the button actually bypasses Q2 E-C. (collector OR). Instead of directly wiring Q2-C to the left of R4, connect it via resistor to its right (Q1-B). This constitutes resistor summation network in Q1 base acting as OR.
No strict necessity however to change anything. Focus on debugging the actual circuit.



Hi all
I've made a latching switch using a transistors but it's not working as it supposed, and i don't know why
When pressing the push button the led is lit but it's not latched since it turns off when depressing the switch
pls help why it's not latching the output voltage?
View attachment 103656
 

Thread Starter

TheNight Silence

Joined Feb 4, 2016
13
thanks for all of u my friends,
thanks plamen, i will check my cct again maybe i did some thing wrong,
thanks gophert i also will try your circuit it seems very simple
 

Phil-S

Joined Dec 4, 2015
140
Do take time to have a look at a CMOS implementation. Designed for exactly this sort of job.
7 days to get a transistor to work is too long, but I understand why YOU need to know it's not working.
CMOS is very forgiving, high impedance, low power. In relay terms, this is known as a "hold-on" circuit, commonly used to control pumps with two float switches and one relay - for what it's worth
 

Thread Starter

TheNight Silence

Joined Feb 4, 2016
13
Hi Phil-s
thanks for your concern,
i have tried to search in google for CMOS logic circuits books or articles and i was bussy for deep search but i failed if you have any links for that i will apperciate
B.R
 

Phil-S

Joined Dec 4, 2015
140
Try this forum for starters
http://www.allaboutcircuits.com/worksheets/latch-circuits/
Book is R.M.MARSTON CMOS CIRCUITS MANUAL. Easily found in Google books, old, but still relevant. I bought a spare copy a couple of years ago. Lancaster might be unavailable in hard copy, but many references on Google - try "book Lancaster CMOS"
You can do almost any latch circuit using single ICs with set and reset buttons
Having said that, I have just been struggling with a voltage translation circuit, and a couple of PNP (2N3906) and NPN (2N3906) transistors were better than the dedicated IC (TXS0101) solution.
Doctronics and Dave's circuits are also useful sites.
Once you've tried a CMOS circuit, you probably won't use transistors other than to boost the output.
 
Top