Delayed ON/OFF circuit

Discussion in 'General Electronics Chat' started by vineet_barc, Sep 4, 2009.

  1. vineet_barc

    Thread Starter New Member

    Sep 4, 2009
    I have developed a hand held battery operated low power ECG unit that operates on 3.3V and sends all the Leads data to a mobile through bluetooth. The ECG report is generated onto the mobile (PNG format) which can be send to the expert immediately through MMS in case of emergency. This is basically for the rural areas where the availibility of good doctors are limited.

    The problem is that during transportation sometime the switch gets ON and all the battery drains off. For this I need some ON/OFF mechanism like we have in mobile where one has to press and hold the power button for 2-3 seconds in order to ON/OFF the mobile.

    Kindly suggest....
  2. kkazem

    Active Member

    Jul 23, 2009
    OK vineet_barc,

    Here's a circuit for you. I'm assuming that you have at least a minimum of electronics knowledge, so here goes. Since I can't take the time right now to draw out a schematic for you, I'm going to describe it such that most any person with minimal to moderate electronics knowledge can make it based on my text here. All resistors are 1/8w unless otherwise specified.

    you'll need a small MOSFET like a 2N7000 or equivalent and a SPST press and release type switch. You'll connect one-side of the switch to B+, also connect all of your EKG electronics plus side to B+, but connect all of the EKG's negative (return) side to the drain of the mosfet. Connect the source of the MOSFET to B-. Connect the open end of the switch to a 1 K OHM resistor and the other end of that resistor to the MOSFET's gate lead. Also, connect a 1uF or 10uF to as high as 100uF electrolytic cap's plus side to the MOSFET gate and connect the minus side of the cap to B- (battery minus). Finally, connect a 2nd resistor of say 10 MEG ohm to start, between the MOSFET gate and B-. The way this works is that the mosfet acts as a switch (I'm assuming that your EKG circuitry is very low current or else you may need a bigger (higher current) mosfet). The mosfet switch needs a minimum of gate voltage to turn-on of about 3.6V. So when you depress the on/off switch initially with a battery hooked up, you'll hold it momentarily such that the electrolytic cap charges up to B+ (9V would work very well, 6V may or may not work well). When you let the switch go, the unit runs until the resistor in parallel with the cap discharges the cap below the threshold voltage of the MOSFET (about 3.6V). It will actually turn off automatically. You can program the on-time for anything from a few seconds to a few hundred seconds (several minutes). If you need long on-times, you should use a CMOS 555 timer like the TI TLC555CN. See the TI website for details. They even have an excel spreadsheet to calculate all the component values you'll need and it draws a crude schematic of the timer circuit.

    Please feel free to contact me with any questions.

    Kamran Kazem
  3. vineet_barc

    Thread Starter New Member

    Sep 4, 2009
    Hi Kamran..
    Thanks a lot for the suggestion. The circuit will work fine for turning it ON. But the problem is that the doctors need minimal manual intervention and due to this it will be not possible to give a seperate ON button and OFF button. I am looking for a circuit which we have in mobile....with press & hold, one can turn ON as well as turn OFF the unit.
    vineet sinha.
  4. flat5

    Active Member

    Nov 13, 2008
    Last edited: Sep 7, 2009
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    Would something as simple as a rigid carrying case work? Place the instrument in it so the activation button can't be pressed. Preserves the warranty, too.