Here are some of the topics getting a lot of notice this week among those posting on our sister site, electro-tech-online.com

Relay troubles — “MY relay circuit is very close to being unstable as the relay chatters or clicks open/shut about once or twice a second at certain times. I have measured the pins on the LM339. Pin 8 is 3.4 V and 9 is 4.8 V. Pin 14 shows 6.45 V. I assume the relay is about 300 Ω and so I expect around 40 mA through the TIP47 but there could be more. The relay is a DPST that switches 240-V mains. It seems the base of the TIP47 is quite close to the limit of turn on/off and if the 7.8-V supply fluctuates (it sees just a drop of 0.1 V at times) by just a little could it cause instability. Not sure how the 5.6-V zener affects the way it works. Any ideas how to fix this?” Read more.

Knight Rider rides again – “I am in the process of making my dash electronics set for a Knight Rider car replica. ….The schematic is an LED sequencer. It receives 12-V and steps down voltage to 4 pairs of led’s, making them light up 1 set at a time. I need to tie into it to run 4 small led bars. each having a set of 3 LEDs on it, 1 per pair in the circuit. I used online resistor calculators and tied in where I thought I would need to, but all that accomplished was lighting up all LEDs at once, bypassing the sequenced startup. I assume the load became too much for the circuit and pulled around the timer. Can someone please help me?” Read more.

Now, let’s all diss switching power supplies — “Computer PSU’s are badly designed Chinese crap – obviously made to be ‘throw away’ components, and as they are so cheap to buy, that’s what they are. I have repaired occasional computer PSU’s, a common problem (as with any SMPSU) was the startup resistors going high – but they are near impossible to work on.” Read more. 

Getting a buzz of a different kind – “I have been fooling with this circuit for a few days and I can’t get an output good enough to power a buzzer. I think it is because of the op amp. Also can anyone tell me what the capacitors  between the op amp and ground do? I’m really struggling to understand the concept of this. 🙁 I don’t really want to just build the circuits. I want to know why and how.  :D” Read more. 

How much does USB suck?  – “USB enumeration is no simple task. For a standardised connector, it really isn’t that standardised! So I decided to start from the basics again and wire up a simple USB circuit to see how much current I can actually pull from my computer without enumeration. So from my Mac mini, with a 13Ω resister in series with the + wire from the USB, I can pull 320 mA. It should be 385 but I guess the current drop is from a slightly smaller voltage than 5 V. This is great, I don’t need USB enumeration. So from what I have read and learned most mains-powered computers will deal with pulling 500 mA no problem without USB enumeration. It would make this slightly less, say 450 mA, to ensure the USB port does not shut down. The problem comes when you start to use a laptop USB port for charging as many of these are limited to 100 mA due to shared USB buses.”.. Read more.