Pilot lights


Thread Starter

Rick Kelly

Hi Gang...

We have an application that is part of our on-line QA procedures. We must take a piece of cheese and weigh it on a static scale. The value from the scale is sent via RS-232 to a SPC system. We must take one sample for every
x number of pieces that exits the wrapping machine.

Sometimes they, the line crew, gets behind in the number of samples that are pending. To let them know how many samples are pending a PLC output flashes a pilot light. The on period is a half second and the off period is two seconds. If, for example, x samples are pending we have an on period of a half second followed by the quarter second off period then the next on period. We then continue this pattern until we have shown x on periods. Next comes the two second off period and then we start over.

The problem is... the bulbs blow out very fast. We have tried using high intensity 6.3 volt LED bulbs instead. They however cost the ridiculous sum of $52.00 EACH and even they fail after a period of time. Therefore... I am looking for a unit that has a two digit seven segment LED display (like something Red Lion would supply) that would take the pulse input from the PLC and display the number of pending samples.
Best Regards... Rick Kelly

Chief Technician
Natural Cuts
Cheese Operations
Kraft Canada
(613) 537-8069 V
(613) 537-8057 F
[email protected]

Gilles Allard

Some manufacturers of Marquee displays (ie large LED displays) offer an option for counting pulses. VORNE is the first to come to my mind (I'm
sure they have it) but you may also want to look at UTICOR

Good luck

Darold Woodward

Most of the time I've driven a display it has been with several outputs in parallel to control the individual segments. Red Lion will have some counters so you'll have to count up and then have outputs to drive the reset.

As an intermediate fix there is also one other option. I recall Sq-D having a light that looked like a bulb and was in the standard diameter size, ran on DC, but contained a cluster of LEDs. This might make an inexpensive intermediate
fix until you get the other on line.

Darold Woodward PE
SEL Inc.
[email protected]

Grenville Spearpoint, Nestle South Afric


Is there any reason why you are not using an automatic check weigher? Depending on your plant set up this could eliminate all plant operator


[email protected]

Johan Bengtsson

I would have approached it in either of the following ways:

1. Use a PIC-processor or something equivalent and output whatever the LED display want.

2. Use a small PLC to recieve the flashing signal and do the same.

I really don't understand how you manage to burn out the LED bulbs however, they should last for a small eternity if they are not feed with a too high voltage. My first attempt (if you really don't need or want the number) would be to put a suitable resistor in series with the LED bulb and making the current go down to approx 50%-75% of the rated current.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
Hi Rick,

Interesting dilemma!.... If I understand the application correctly, I can't think of any "off-the-shelf" units that would count pulses and display the total in a way that would be appropriate for your situation. First off though, I would investigate WHY the bulbs fail so quickly, especially the LED types. These are supposed to last a L-O-N-G time! (I think that's one of the reasons why they're so expensive)

I used to use a similar "flashing light" to display fault codes on small machines, but the cost of PLC displays is so cheap these days, it's easier just to install one of these rather than using the pilot light approach.

Most likely, the pulsed output is getting it's data from a single counter in the PLC. Depending on the brand of PLC, maybe you could buy an "aftermarket" display that can directly read the current value of the counter in the PLC. This would require no reprogramming of the PLC to add this display. Another approach would be to buy an inexpensive PLC and text display (Automation Direct comes to mind!). Just connect the output of the existing PLC to the input of this new PLC, and program it to display the
count value. This will cost you less than US$400 for everything (yes, this includes the programming software as well!).

Good luck,

- Eric Nelson
[email protected]
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA

P.S. You mentioned needing a TWO digit display..... Does your QA department actually fall far enough behind that you need to count higher than nine? <grin>

Pierre Desrochers

The cheapest way to fix this problem is to use a 250 Volts 500 watts bulb, just like the ones use in your house but higher volts and Power. Connect
this to and 120 volts circuit and with this 125 watts results it will last forever.

We do also this when excessive vibrations break the elements...


Ralphsnyder, Grayg

1 - Add an inline weigh scale to do the work. Interface it thru your PLC then pass it onto your SPC system. Most have an analog output option that you can use. Then you could weigh every single piece of product if you want to. This will also eliminate questions about how well your quality inspection system is working in this area.

2 - Find a different supplier for the LED bulbs... that price is ridiculous.

3 - If LEDs are 'burning out' then there is an application problem.
a - For a FIXED voltage you need to select the proper current limiting resistor to put in series with the LED.
b - You need to limit the maximum voltage to the LED so that the current limiting resistor selected can properly protect the LED. You could
use a diode in series with the signal to feed the LED and also install a zener diode across the LED/resistor to limit the voltage.

Signal (+)-----|>|-----(node 1)-----/\/\/----------|>|-----(node 2)-----(-)

(node 1)-----(Zener Diode)-----(node 2)

The LED is between the resistor and (node 2).
The diode between the signal and the resistor is a small signal diode. The resistor between the signal diode and LED is to limit current. The zener diode is to limit the maximum voltage to the LED.

W Grayg Ralphsnyder
Process Control Engineer
Weyerhaeuser EOSB