Connect with us

Need help with strobe circuit

Discussion in 'General Electronics Discussion' started by Alex, Mar 9, 2012.

Scroll to continue with content
  1. Alex

    Alex

    7
    0
    Mar 9, 2012
    I'm new to electronics and am trying to build a strobe for my RC airplane and it's not working. I want a double flash with a variable rate between flashes. Found a basic 555 timer circuit and tried to modify it. The first 555 is set up to be about 1 Second on and 1 Second off. During the 1.5 Second ON, it feeds power to the second 555 which is set up to rapidly flash the LED. So basically the second 555 is being turned on and off at 1 second intervals and while ON, it blinks the LED. Also incuded a 1K pot to vary the rate between double flashes. Could really use some help as to what I'm doing wrong. Thanks.
     

    Attached Files:

  2. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Timer2 Pin8 should not be connected to pin3 of Timer1. Timer2 pin8 must be connected to +5V. Connect 0.1 mfd to pin or CV of 2 timer to ground.

    [​IMG]
     
  3. Alex

    Alex

    7
    0
    Mar 9, 2012
    Sorry, not following you. Timer2 pin8 is connected to +5V supply as per diagram and has no connection to pin3.
     
  4. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010

    Not in that schematic you posted!
     
  5. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    This was your diagram that you posted. I mark with red line for correct connection.
    [​IMG]
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    edit (again) these comments refer to the corrected diagram, not the original. Rleo6965 has retained much of your original layout

    When you draw a circuit nbsiqeqimu it often confuses people.

    Convention is to have (in your case) +5V at the top and ground at the bottom.

    You don't actually show ground explicitly (oh, yes you do -- well hidden!), but I'm pretty sure it's the like at the top.

    Similarly, your +5V comes in at the side and winds its way around the bottom.

    Apart from that, the circuit seems correct.

    I'd suggest that you have a potential problem with the low frequency oscillator in that the variable resistor is in parallel with another resistor (perhaps you meant to put them in series?). If you reduce this to zero, the V+ is connected to the discharge pin which will cause excessive currents to flow.

    Again, for the low frequency oscillator, I would recommend a smaller value capacitor and higher value resistors. This will reduce both the current consumed by the device and allow for smaller components. This observation also applies to the second oscillator.

    Consider having resistors in a range of about 10k to 220k and see what capacitors get selected by one of the many 555 calculators on the net.
     
    Last edited: Mar 10, 2012
  7. Alex

    Alex

    7
    0
    Mar 9, 2012
    Sorry guys, as I mentioned I'm not that versed in electronics and that schematic more than likely does't correspond exactly to the PCB. The attached is the actual PCB I used. Would appreciate any insight as to what is wrong with it. Thanks!
     

    Attached Files:

  8. Alex

    Alex

    7
    0
    Mar 9, 2012
    Are these the main corrections to the PCB that you indicated?
     

    Attached Files:

  9. Alex

    Alex

    7
    0
    Mar 9, 2012
    This is the strobe effect I'm looking for.
     

    Attached Files:

  10. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Did you try building this circuit on a breadboard first? One thing that was mentioned and you still have wrong on the pcb is the possible short from pin8 to pin7 of the first timer when the 1k pot. is 0ohms. The 1k pot. should be in series with the 120Ohm resistor. I also don't see the ground connection for the LED or it's series resistor???

    The problem that Rleo pointed out is fixed on the PCB.
     
    Last edited: Mar 10, 2012
  11. Alex

    Alex

    7
    0
    Mar 9, 2012
    Unfortunately I don't have a breadboard to try this out on but can modify the PCB. I've removed the 1K pot till I can get the strobe working and than will install it in series with the 120Ohm resistor. I've also indicated the LED connection on the PCB.
     

    Attached Files:

  12. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I wonder if a pull-down resistor on pin3 of timer1 will have any effect. Say a high value like 10k

    What happens if you remove timer2 completely? does timer1 work as designed?
     
  13. Alex

    Alex

    7
    0
    Mar 9, 2012
    Got a hold of a breadboard and played around with this schematic last night. Could not get it to work, so am going to start from scratch. The attached schematic is similar, with two 555's. Timer1 will put out pulse t1H lasting 800ms withe t1L of 1200ms. The t1H will trigger timer2 which will output 2 pulses of 250ms duration during the t1H input. my question is do I simply connect timer1 pin3 to timer2 pin4?
     

    Attached Files:

  14. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    If you are starting from scratch, consider a 4093 oscillator (one resistor and one capacitor) driving a 4017 decade counter, using two outputs for your double flash.

    This may do what you want.
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Here's my suggestion.

    [​IMG]

    The 555 produces a rectangular output wave with a period of about one second. Each time the output changes state, C3 charges up or discharges, and there is a burst of voltage across R3 as it does so. The four diodes form a bridge rectifier that converts that voltage pulse from bipolar to unipolar so the LED(s) will light on each transition.

    R4 limits the LED current. The LED (with R4) is almost in parallel with R3, and the LED current has a bigger effect on the pulse duration than the value of R3.

    The timing of the blinks is determined by R1, R2 and C1. The short time between the two blinks is proportional to R2 and the time from one pair of blinks to the next is proportional to R1+R2. Both are proportional to C1.

    C2 is a decoupling capacitor to help the 555's stability. CDRIVE, I've taken your advice on this one :)

    Reducing R4 will increase the peak LED brightness but also shorten the blink duration, so the apparent brightness may not change much. Increasing C3 will increase the apparent brightness though, by increasing the blink duration without affecting the peak current. With the values given, the time for the LED current to fall from its peak (~100 mA) to half (50 mA) is about 1.4 ms. Watch the peak current specification of your LED; LEDs can be damaged by overcurrent. Also, the 1N914 diodes and the 555 are only rated for 200 mA so limit the peak current to that, or use a buffer and bigger diodes. If you need more apparent brightness, your first change should be to increase C3.

    Varying R3 will make little visible difference. Decreasing it significantly will shorten the blinks and reduce the peak LED current. Increasing R3 will slightly lengthen the blinks and slightly increase the peak LED current, but may cause the end of the blink to become less clearly defined and look more like a fade-out than an abrupt cut-off.

    You can connect multiple LEDs in series, but if the total forward voltage becomes significant (more than about 3~4V at 12V supply) the current will start to drop noticeably and you will need to reduce R4. I've recommended a 12V DC supply; up to 15 is OK but if you use less than 9V you'll need to reduce R4 and the pulses may become less clearly defined, so I don't recommend using less than 9V.

    The part number for the LED shown on the schematic has no significance. LTSpice has a very limited range of LEDs so I just chose one that looked kinda suitable for my simulation.
     

    Attached Files:

    Last edited: Nov 9, 2012
  16. trondyne

    trondyne

    63
    0
    Oct 17, 2012
    That's a cool circuit.. The application though calls for a 5V supply and more current, as this is for an RC plane application... This is also what I need.. Any ideas for modding this circuit for 5V and double or more current? This would normally drive at least 2 LEDs. Not sure what is meant by "a buffer".
     
    Last edited: Nov 10, 2012
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    [​IMG]

    This circuit runs from 5V. The positive/negative pulse circuit is based on another design I did for an electronics point user, to simulate a heartbeat. The 2N4403 drops about 0.2V so you can calculate the series resistor value from (Vsupply - 0.2 - Vled) / Iled. The pulse is about 10 ms long which should give good apparent brightness.
     

    Attached Files:

  18. trondyne

    trondyne

    63
    0
    Oct 17, 2012
    Wow, looks good.. I will get that on the board as soon as I can get the parts..

    Thanks!
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Wow, another Kris Blue original! I think I'll call it "CircuitusCleverusMaximus". ;)

    Kool Suff!
    Chris
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-