BJT Invertive switching logic question - Page 1
Forum section: electronics / projects

author: john23
author_rank: Frequent Contributor
timestamp: August 31, 2024, 03:02:55 am
content: As you can see in the photo from LTSPICE and the graphs, I have tried to pass the voltage from P12 only when M12 signal is going from 0 to -12. However my Q1 is always conducting and Q3 is always conducting.Is there a way to close Q1 and Q3 when M12 is 0 and open Q1 and Q3 when M12 is -12V using a BJT logic?Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2024, 03:43:20 am
content: When asking questions about a LTspice sim, always attach your .asc file! For more complex LTspice simulations using 3rd party symbols and model files, it is best to put all the 3rd party files in a folder of their own, together with the .asc file and zip them to post here. Don't include LTspice auto-generated files (e.g. .log .raw ...) as they can get very big and will bloat the zip file.Your M12 signal doesn't do what you intend - due to the connections to V2 it actually pulses from 0V to +12V.How best to tackle this problem depends on whether minimising the parts count is a priority, whether or not you have other fixed (non-pulsing) supply voltages available, the permissible load on the M12 signal, how sharp a transition between Q1 on and off you require, and the required tolerance on the M12 switching threshold. e.g. what is the largest negative voltage it should NOT switch on for and what is the smallest negative voltage it MUST switch on for?

author: john23
author_rank: Frequent Contributor
timestamp: August 31, 2024, 09:46:49 am
content: Hello Ian, I have attached the circuit but its tottal garbage.basicly i am trying to implement the following state machine.COuld you please give some simular circuit which does similar this as the functionality of the state machine with BJT's so i would lear the BJT logic of implementing things?Thanks.

author: Andy Chee
author_rank: Super Contributor
timestamp: August 31, 2024, 11:32:51 am
content: I would take your -12V and run it through an optocoupler LED. The transistor of your optocoupler can then switch the PNP pass transistor of your +12V supply.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 31, 2024, 12:08:47 pm
content: Quote from: john23 on August 31, 2024, 03:02:55 amDon't you see at your chart that your M12 is +12V?Quote from: john23 on August 31, 2024, 03:02:55 amIn your other thread you had a circuit that switches Q1 on when M12 is -12V.What do you call BJT logic?I haven't come across this term in past. My first understanding is BJT logic = TTL logic. So it looks that you are asking about using 7400 serie circuits family. These are circuits using BJT to get logic gates, flip-flops, registers, counters, and so on.

author: ledtester
author_rank: Super Contributor
timestamp: August 31, 2024, 01:25:54 pm
content: Is this the basic idea of what you are looking for?(Falstad Simulation)You can adjust the P voltage with the slider on the right side and switch the voltage of M by clicking on the switch.Note that it makes use of a -12V supply.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2024, 01:55:05 pm
content: Here's another variant of ledtester's circuit.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2024, 02:53:10 pm
content: Here's a circuit that will perform the desired function without needing any permanent (DC) positive or negative supplies. However, it wont work with sub-microsecond pulses, or without a load resistor (as in John23's original post). I've also removed the 1nF capacitor he had decoupling the switched output because it was degrading the fast pulse response.