Connect with us

Driving an Optocoupler from 74LS Series Logic

Discussion in 'Electronic Design' started by GigaNews, Jan 1, 2004.

Scroll to continue with content
  1. GigaNews

    GigaNews Guest

    I am working on a project for a friend of mine. This device will switch
    between N different electronic instruments and send the output to a single
    output. In thinking about the design for this, I was going to use relays,
    but then found optocouplers to fit our needs better (no noise, spikes, pops,
    and isolates from logic circuit). So from Silonex, I ordered the NSL-32SR3
    (, which offers low
    closed resistance (60 ohms) and high open resistance (25M ohms). However,
    to driver the device, I need 20mA at 2.5V.

    I am using a J-K flip-flop (74ALS109A) to hold each channel's toggle state
    (on/off). I have a 5 volt power supply for project.

    The question is, how do I interface the Q output of the flip-flop to the
    optocoupler? Can I drive it directly from the Q output pin?

    Thanks in advance!
  2. John Larkin

    John Larkin Guest

    Better is


  3. James Meyer

    James Meyer Guest

    Most TTL compatable output pins have better pull-down capability than
    pull-up. Connect the opto from the 5 volt supply to the output pin and it
    should work. A low on the FF output will select the opto.

  4. GigaNews

    GigaNews Guest

    I should add some specifics...

    This optocoupler consists of an LED input optically coupled to a photocell.
    The photocell resistance is high when the LED current is "off" and low
    resistance when the LED current is "on". See

    So the circuit I want looks like this:

    need 20mA @ 2.5V here
    | J-K |
    V +-------+
    | Q | --------- circuit
    here --------------- ----------- | NSL |----------
    | | what to do?
    LED side | 32SR3| photocell side
    | |
    +-----------| |----------
    | +-------+

    [other inputs/outputs on J-K left out]

    The photocell side is where I'll hook in the input from the instruments into
    the common output.

  5. Luhan Monat

    Luhan Monat Guest


    I made such a device for a stereo store back in Urbanna years ago. I
    used CD4051's. Put +5 for VDD (as usual) then put -10 volts on VEE.
    Control the selection (1 or 8) with the 3 address lines. These chips
    have only about 200ohms in the on channel.

    Note: do not use +/- 5 volts for the supplies. These devices have a
    small 'crossover' notch at the mid voltage point. Using the -10 for VEE
    moves the notch out of the way for low signal levels.
  6. ULN2003 or cousins. Only catch is it inverts so you use not Q.
  7. Fred Bloggs

    Fred Bloggs Guest

    What is the worst case OC output terminal resistance for the NSL-32SR3
    across all LS09's, 500 ppm/oC power supply variation, device
    characteristics, 0-70oC operating temps, and OC LED aging degradation
    over 100,000 hours operation? Just sketch an approach if you can't come
    up with hard numbers- take your time, you have five minutes:

    Please view in a fixed-width font such as

    | |
    | |
    / /
    2.2K 120
    / /
    \ \
    | |
    | c
    | |/
    LS109 Q >-----+----| 2N3904
    OC ---
    LED \ / ~
  8. John Larkin

    John Larkin Guest

    Your circuit dumps all available TTL pullup current *plus* the current
    through the 120 ohms, *plus* the 2.2k pullup current, all funneled
    into the LED... 45 ma maybe, but not very well defined. That much
    current will make it degrade in a lot less than 100K hours.

    That's a lot of parts just to damage one optoisolator.

  9. Fred Bloggs

    Fred Bloggs Guest

    Nah- the LS/ALS fades to less than 0.1mA at Voh>3V, the OC LED is Vf
    2.5V @ If=20mA, and the transistor saturates to Vce~0.1-0.2V. The
    circuit has far better defined current than your direct drive. Better
    luck at your next interview-
  10. Hmm... they must be using a visible LED, perhaps a green one. That
    would make sense, IIRC, CdS peaks at wavelengths close to where the
    human eye does. Note that the 2.5V is a MAXIMUM figure, not a typical

    Best regards,
    Spehro Pefhany
  11. Fred Bloggs

    Fred Bloggs Guest

    Yes- the CdS wavelength response is very similar to human vision-and
    this accounts for its popularity in light detection applications. The
    2.5V should be Typ Vf, the datasheet is obviously in error-using W for
    Ohms-and spec'ing 4mA reverse current at 4V- where they mean uA @100oC.
  12. John Larkin

    John Larkin Guest

    LS parts aren't spec'd to pull up hard, but they are actually pretty
    good, and get better at max Vcc and temperature. There's nothing
    between Vcc and an output but a darlington fed by a pullup. And the
    LED here has a *max* drop of 2.5 volts, with no min specified. If
    things stack up, this circuit could indeed push a *lot* of base
    current. The OP wants 20 mA but opto spec sheet says it can stand only

    If you really want to use a bipolar transistor, this is a lot

    ttl-----------b pnp

    or, for one more part,

    ttl-----r-----b npn

    which is less sensitive to diode Vf variations.

    As far as the interview goes, I already own the company.

  13. I read in that John Larkin <[email protected]> wrote (in <[email protected]>) about 'Driving an Optocoupler from 74LS Series Logic', on Sat,
    3 Jan 2004:
    I just bought your bank. Now, about this interview.... (;-)
  14. John Larkin

    John Larkin Guest

    Don't give me any lip, young man, or we'll withdraw all our accounts.

  15. I read in that John Larkin <[email protected]> wrote (in <[email protected]>) about 'Driving an Optocoupler from 74LS Series Logic', on Sat,
    3 Jan 2004:
    That should increase our liquidity by a significant factor, although it
    will impact our cash flow. Have we really conned^H^H^H^H^H^H negotiated
    you into paying THAT much interest? (;-)
  16. John Larkin

    John Larkin Guest

    Sorry, I'm old fashioned. We never borrow money and pay for everything
    in cash. We save money, and pray for high interest rates.

    The problem with borrowing money is that you have to pay it back.

  17. Fred Bloggs

    Fred Bloggs Guest

    They must have re-designed the family because there used to be something
    like 100 ohms in the collector of the pull-up. I checked the TI logic
    family handbook for the LS/ALS family and the output characteristic
    curve is a total fictional joke- LS couldn't pull a unit load above 3V-
    and by their graph the stuff can drive 20 ohm lines to full logic
    K- that's better- you can count on LS staying on the low side.
  18. I read in that John Larkin <[email protected]> wrote (in <[email protected]>) about 'Driving an Optocoupler from 74LS Series Logic', on Sat,
    3 Jan 2004:
    Quite agree. I run my firm on the same basis.
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