Old 09-26-2011, 08:07 PM #1
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Default Designing a breakout circuit for Flexmods, one question...

Hello,

I have 6 Flexmod drivers that I want to hookup to a breakout board circuit that I am designing. The circuit is very simple but I have a question concerning one design feature that I came up with.

I want to implement a SPDT switch that will select between enabling the TTL modulation input on or off for each driver. I have this design (shown below, for one driver only) that incorporates an indicator LED that turns on when the modulation input switched on:



Only problem with this setup is that the LED will drop the voltage of the TTL input voltage (5 volts) before it is fed to the driver. To correct this I included an opamp at the TTL input that will correct for the voltage drop of the LED:



Is this the best way to implement this feature or can it be improved upon? One thing that concerns me is that the opamp will not supply enough current to power the LED and provide the modulation. Am I correct in this assumption? I am fairly new to circuit design but this is what I came up with.

Any advice is appreciated.


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Old 09-26-2011, 08:26 PM #2
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Default Re: Designing a breakout circuit for Flexmods, one question...

Tap off the 5Volt supply and use a CMOS OR TTL Hex Buffer..
The Hex Buffer will not draw any significant current and can
easily drive an LED.. (CD4584 )
The only problem I see is that the LED will want to follow
the duty cycle of the TTL signal to the Flexmode.

OR easier still... use a Double Pole Double Throw switch using
the extra pole for the LED indicator circuit.


Jerry
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Old 09-26-2011, 09:02 PM #3
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Default Re: Designing a breakout circuit for Flexmods, one question...

Quote:
Originally Posted by lasersbee View Post
Tap off the 5Volt supply and use a CMOS OR TTL Hex Buffer..
The Hex Buffer will not draw any significant current and can
easily drive an LED.. (CD4584 )
The only problem I see is that the LED will want to follow
the duty cycle of the TTL signal to the Flexmode.

OR easier still... use a Double Pole Double Throw switch using
the extra pole for the LED indicator circuit.


Jerry
The DPDT switch seems to be the better idea. Now if only I can find some that are small enough to fit on a protoboard. Are their any DIP variety switches on this kind?
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