How to mimic a signal. - Page 1
Forum section: electronics / projects

author: caius
author_rank: Regular Contributor
timestamp: June 05, 2015, 08:32:58 am
content: Hi all,I repair electronics for fun and hobby (and why not, also for profit).My primary field is arcade machines, you can read more about onwww.jammarcade.netSome days ago I came across a PCB which require alternate current (usually arcade boards require standard voltages like +5V, -5V and +12V), specifically 6-6.3VAC in this case.This voltage is required by a timer circuit (with a NE555, a 74LS193 and a 74LS74) which has to generate a RESET signal for a custom IC (a protection MCU).Obviously if the voltage is not supplied, the circuit is not activated and the custom IC never reset itself with the result that the game keeps resetting in a andless loop.Obviously, If you use a standard 220VAC to 6.3VAC , the game works fine.Now my question is : is there a way to mimic this signal without the need of using an external transformer (they are big and and impractical)?As you can see from attached schematics the 6-6.3VAC goes trough a 1S1588 diode (so it's half rectified), then trough a 47K resistor before reach the PIN2-6 of the NE555.I attach as well pictures of the waveform observed (with an analog scope set to to 5 Volt per division, time is on 2mS for division) on the cathode of the 'D5' diode and on pin2 on the NE555.I was thinking about a little function generator or some circuit made with ArduinoAny help or suggestions will be greatly appreciated.Thanks.

author: dom0
author_rank: Super Contributor
timestamp: June 05, 2015, 08:57:43 am
content: Just use another NE555 to make a square wave of suitable frequency ... or hardwire the output of the 555 to +5 V.

author: Rerouter
author_rank: Super Contributor
timestamp: June 05, 2015, 10:00:39 am
content: http://www.indiabix.com/_files/images/electronics-circuits/555-square-wave-generator.png?6V AC corresponds to 8.5V DC, its being pulsed at either 50 or 60Hz, so easy to hook up the original 555 to replicate the output,http://www.indiabix.com/_files/images/electronics-circuits/555-square-wave-generator.png

author: caius
author_rank: Regular Contributor
timestamp: June 05, 2015, 10:12:16 am
content: Quote from: Rerouter on June 05, 2015, 10:00:39 amThanks for your simulation.Yes, signal is 50 Hz (I'm located in Europe).Just a curiosity : which software did you use for the simulation?P.S.Can I use some diode in series (or a voltage divider) to drop the +12V to 10V (the latter is not standard arcade voltage)?

author: dom0
author_rank: Super Contributor
timestamp: June 05, 2015, 10:27:44 am
content: It's somewhat independent of supply voltage ; 12 V will be fine.If you use a NE555 (bipolar), add beefy bypassing caps (at least 1 µF ceramic + some electrolytic). The CMOS 555 versions are less demanding there.

author: caius
author_rank: Regular Contributor
timestamp: June 05, 2015, 01:44:17 pm
content: I just tried the circuit posted above using another NE555 (not the one on the PCB) and it works perfectly.I post some pictures.Many thanks, guys.P.S.Later I will compare on scope original and reproduced NE555 output and post results here.@dom0 : I use one ceramic 100nF bypass capacitor on the NEE55 and it seems it does the job.Lastly : is it possible to use +5VDC intead of +12VDC?

author: dom0
author_rank: Super Contributor
timestamp: June 05, 2015, 02:47:26 pm
content: Yeah, the 555's work from about 4.5 V to 15 V. But you might need to adjust R46/R44 to get a high enough level to the other 555.If you're using only 100n I'd suggest to check the supply voltages directly on the 555, they switch rather fast (about 30 ns tr/tf on the bipolar versions) and with quite a lot of shoot-through - don't be tricked by low average current consumption.

author: caius
author_rank: Regular Contributor
timestamp: June 05, 2015, 02:55:34 pm
content: Quote from: dom0 on June 05, 2015, 02:47:26 pmOK.Well actually something of strange happened.The PCB boost also with original circuit , with or without the NE555.I can't explain this.I have only noticed some smoke form the circuit I built (I immediately cut off power).NE555 didn't get damaged but was very hot.Probably the 100Nf bypass capacitor wasn't enough.P.S.I've used the posted circuit in replacement of the original one and not coupled with this.I completely removed (by socketing it) the original NE555 on the PCB