TTL Pull-down and flyback diode on coil-relay

Thread Starter


Joined Oct 7, 2011
I have a few questions regarding TTL control of Opto-relays, which in-turn switch Inductor-Coiled power relays.
I am utilizing the TTL outputs from a Frame-grabber card to control digital signals for Camera/Lamp/Alarm circuits through Opto-Isolated relays. I am set up for postive logic (5VDC TTL high turns the circuit ON). All 3 opto-relays toggle 24VDC to the field. The Camera and Lamp circuits drive additional coil relays in the field to isolate High AMP 24VDC (8A) and 120VAC Line voltage to the devices. The Alarm circuit switches Low Amp 24VDC directly connected to the output of the opto-relay.

Frame grabber - National Instruments PCIe-1427:
Relay - Invensys IO-ODC-R0-060:

I setup my initial circuits, confirmed that TTL outputs respond high/low appropriately (indicator LEDs on opto-relays light on/off correctly), and the Alarm On/Off works fine, but am experiencing some glitchy field response switching the Camera and Lamp inductor-coil power relays.

I have researched in other forums and think I know what I need to do, but I am looking for outside confirmation before I buy/install parts and give it a go.

Q1) When the card goes inactive it appears that all 3 TTL outputs float at ~1.2VDC (the LEDs glow dimly), and I think I need to include pull-down resistors to bring the TTL signals low. Does this sound reasonable? What would be an acceptable Resistor value 1K, 5K, 10K, higher? I would think higher resistance is better, to a point, to minimize current draw on the TTL output from the card.

Q2) The Camera works sometimes, but I have seen intermittant response at the Coil-Relay. The Lamp works when it is first turned On and Off, but the Coil-Relay won't turn back on during subsequent attempts (The coil relay won't re-engage even though the opto-relay is toggling on/off). I think I need to install a flyback diodes across the coils. What would be an appropriate diode (remember, there is 24VDC across the coil)?

panic mode

Joined Oct 10, 2011
not sure i understand correctly:
you are measuring output voltage while LED is connected as load?
if the measurement is with load you are probably using TTL outputs incorrectly.

remember, all TTL outputs have NPN transistor pulling output low when output is zero state. when transistor is off, resistor pulls the output high. few chips have open collector meaning you need to add external pull up resistor.
if the outputs are used to drive inputs of other gates (very little load), this is not an issue but if you are trying to connect something that draws several mA you will get poor results.

better way is to use some sort of buffer or if no buffer or drivers is used, at least connect your load from TTL output to 5V rail. this will reverse logic (load is on when TTL output is low) so you would probably want: