Sound recording device (problem with 555 in monostable)

Discussion in 'General Electronics Chat' started by Sardaukar, Jun 19, 2009.

  1. Sardaukar

    Thread Starter New Member

    Jun 19, 2009
    6
    0
    Hello everyone,

    I am building a device which allows to record short messages on a chip from a PC and then play them by giving voltage pulse to the device. One part of it uses a 555 timer in monostable mode. When an audio file is played on a PC, input of an optron detects a first pulse and shortens trigger pin (2) of 555 to ground and output pulse of approximately 8-10 seconds is given to the recording chip (it needs the pulse to know when to record) via another optron.

    The problem is that when 555 triggers, the output pulse never ends. I placed a LED from pin 3 (output) to ground in series with resistance to monitor when timer is working, and the only way to turn it off is either by removing supply voltage or shorting reset pin to ground. I observed input signal and threshold voltage on oscilloscope, input signal always stays high unless I manipulate is, so problem is not retriggering. The threshold voltage, however, rises to the point when it is bigger than 2/3 of Vcc and stays the same (no discharge through pin 7). I tested the chip on a breadboard with same components without the rest of the circuit and it works fine. I also read on this forum that sometimes it can happen because of leakage current from electrolytic capacitor, so I changed charging capacitor to tantalum, no result.

    Now the weird part. If I touch pin 2 (input) with a wire (other end of the wire is not connected to anything), the LED lights up (output is high). When the output is low, shorting reset pin to ground can also make output high.

    I have very little experience with electronics (I am still afraid of transistors =)) ), so behaviour of the timer is really confusing. I suspect some difficult stuff like leakage currents and parasitic voltages is involved, and since I couldnt find anything about this problem on the internet (and Ive read everything Google gave about 555, including this forum), I'd be glad if someone could help me with this.

    The schematic is only for the 555 part, however I have the rest of the circuit on same supply and ground rails as the timer, so I can provide the whole circuit if necessary.

    Thanks in advance!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Pin 2 is connected to the Vcc, why?
     
  3. Sardaukar

    Thread Starter New Member

    Jun 19, 2009
    6
    0
    Pin 2 is normally high (connected to Vcc), and when I need an output pulse I short it to ground (to have a negative pulse into trigger pin and get output high).
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    If you short pin2 on that schematic you also short the +5VDC power supply.
     
  5. Sardaukar

    Thread Starter New Member

    Jun 19, 2009
    6
    0
    Thank you for your response.

    The schematic attached in the first post is not entirely accurate because when I removed the rest of the circuit from it, I also accidentally deleted 1k resistor between pin2 and power supply, so Vcc is shorted to ground through the resistor.

    But I believe if that was the case, the 555 simply wouldnt work, instead of displaying odd behaviour described in the first post.
     
  6. Sardaukar

    Thread Starter New Member

    Jun 19, 2009
    6
    0
    Ill ry CMOS version of 555, maybe that will improve the situation.
     
Loading...