vajirkar

Audioguru, Yeah... that's the rest of the circuit. Thanks for the techniques/precautions. About the complex "flip-flop" :) , I'd love to hear comments and critical and otherwise. How can I improve this? I would have just loved to avoid the mess by programming the microcontroller myself. I just don't have all the tools for that. Maybe building a programmer may be a good investment in itself. In anycase comments are really welcome! - Sid

I tried to narrow down the problem and it seems to be related to length of the wires to the limit switches. I simulated the limit switches by a pair of jumpers on the board and the latching is much more reliable. So its possible that in rare circumstances the relay-switching noise does affect the circuit but its probably not the root cause here. With the same values of resistors (10K, 1M) I experimented using a darlington pair with hFEmin = 25000. The problem still persists. I'll try hotwaterwizard's mod when I get a chance. - Sid

I was thinking of adding an extra transistor driver between the AND gate output and the limit switch. Dunno if its going to help. - Sid

I have a latch driving a transistor (indirectly through a few gates). The transistor is used to drive a relay that drives a 110V linear actuator motor. When the relay is not connected to the actuator motor, the latch works perfectly. A single button click is enough to set the latch, the reset button then resets it. When the relay is connected to the actuator motor load, the latching is less reliable. Sometimes multiple button clicks are needed to set the latch. Another point worth mentioning is that connection from the AND gate to the Limit Switches (LS1 and LS2) is a little far. About 2 feet of twisted wires connects the output of the AND gate to the Limit Switches. I'm not sure if its the load that's causing this or the length of the wire. BTW I have replaced the CMOS AND gate with LSTTL, but that did not help the latching issue.

Thanks for all the detailed info guys. I tried all the different techniques, but the main problem seems to be that sudden switching of the large inductive load isn't a great idea. All I actually needed was a way to stop the actuator motor, so I just added an extra button to switch off both transistors and stop the actuator. I have another question. I'll just start a separate thread for that. Thanks again, Sid

Let me add that the load is a motor of a linear actuator. Could it be possible that its not good to suddenly change the direction of rotation for a motor (inductive) load? Maybe there is nothing wrong with the transistor switching circuit at all! - Sid

GPG.. yes that's exactly what I have done, and that's why I don't have spare contacts. For some reason I still see a spark. See the attached image. MP, you are probably right. I did have 100K and 1M there first I may have made a mistake in switching to 10K. I'll try using the 10K and 100K as audioguru suggested. - Sid

This may have been discussed here before, so apologies for any repetition. Has anyone used the services offered by www.pad2pad.com? The layout drawing tool seems nice, but how is their manufacturing and pricing? - Sid

Thanks, nice explanation audioguru. Thanks again for the suggestion. I'll give it shot over the weekend. - Sid

kingratman, I was trying to build a IR receiver for a mini-project and in the process found a bunch of IR transmitter (remote control) circuits on the net. Just google for IR remote circuit, you will find tons of them. BTW if anyone is looking for a simple IR receiver check rentron.com. Actually they also have some IR remote control stuff there. - Sid

GPG, I don't have any spare contacts on the relays. If I did, what did you have in mind? Just curious. Audioguru, I like that idea. Why is the diode at the emitter needed? The discharge will occur from the diode at the base, no? Thanks for your responses, Sid

Hi guys, I have a simple transistor switch driving a relay. See the first figure (without diode D1). I have two such transistors driving two relays. The relays switch a 110V load. Ideally only one relay should be switched on at a time, so only one transistor should be switched on. The driving circuit seems to be doing this correctly, however, when switching from one transistor to the other (i.e. switching from one relay to other), the transistor being switched off is probably not switching off fast enough. So there is a small window in time when both relays could be switched on. I'm thinking about using a diode D1 at the base of the transistor as in figure (with diode D1) to speed up the switching off process. I haven't tried it (hopefully this weekend), but I wanted to know if this could lead to some other problem that may not be obvious to me.