Hold reading on LCD meter readout

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
Karl Cohen
Posts: 2
Joined: Mon May 26, 2003 1:01 am
Contact:

Hold reading on LCD meter readout

Post by Karl Cohen » Mon May 26, 2003 1:37 pm

Can anyone out there suggest a way to add a "hold" feature to my electronic footcandle meter with LCD display? In dim light it is impossible to read the display and it is impractical to use a flashlight to check the reading.

russlk
Posts: 563
Joined: Thu Dec 06, 2001 1:01 am
Location: New Hampshire
Contact:

Re: Hold reading on LCD meter readout

Post by russlk » Mon May 26, 2003 6:36 pm

A "grain of wheat" lamp mounted right over the display should give enough light to read it. You could build a shade around it so no one else can see the light.

gadgeteer
Posts: 102
Joined: Fri Mar 07, 2003 1:01 am
Location: Ok!
Contact:

Re: Hold reading on LCD meter readout

Post by gadgeteer » Tue May 27, 2003 4:10 am

Electronic Goldmine is currently selling WHITE SURFACE-MOUNT LED's for 49¢ each...<p>www.goldmine-elec.com<p>To add a HOLD button, we would need to know the circuit of your meter; you can't simply kill the CLOCK on the voltmeter chip, 'cause the display is almost certainly MUTLIPLEXED (so only certain segments would remain), and besides that would give the LCD a direct current component; DC destroys LCD's...<p>[ May 27, 2003: Message edited by: gadgeteer ]</p>

Chris Foley
Posts: 146
Joined: Mon Jan 13, 2003 1:01 am
Location: Chicago IL
Contact:

Re: Hold reading on LCD meter readout

Post by Chris Foley » Tue May 27, 2003 6:14 am

On the hobbyist level, and if you'd like to try something different, you might want to take a look at doing a home-brew footcandle meter with a Cadmium Sulfide photocell and an ohmmeter. If you have a DMM with a measurement hold function, you'll be able to get the measurement you need.<p>The CdS photocell has a wide sensitivity range similar to the human eye, and are usually factory tested with incandescent light bulbs. That makes them good for measuring artificial lighting in a room. The challenge is that the footcandle-resistance curve is non-linear. But since you already have a footcandle meter, you can "calibrate" the CdS photocell by measuring the intensity of a light source simultaneously with your meter and the photocell-ohmmeter, and build up a footcandle-resistance chart. You can then measure the resistance of the CdS photocell at any time, use the measure hold function on the DMM, and then do the math.<p>You shouldn't expect better than a couple percent accuracy on this one -- it'll do in a pinch, though, if you don't need too much precision or accuracy. Also remember that these respond slowly to light, and you'll have to wait several seconds before pressing the hold button on the meter to allow the photocell to stabilize. Actually, they've become less popular over time for that reason -- they're slow.<p>Radio Shack sells a CdS photocell as 276-1657.<p>Radio Shack Cadmium Sulfide Photocell<p>Here's a reference on a H.S. physics experiment using a CdS photocell<p>Equipment Construction: An Inexpensive Spectrophotometer<p>[ May 27, 2003: Message edited by: Chris Foley ]</p>

bruinbear714
Posts: 26
Joined: Thu Apr 17, 2003 1:01 am
Contact:

Re: Hold reading on LCD meter readout

Post by bruinbear714 » Tue May 27, 2003 8:58 am

You can do it by slowing down the sample rate of the system by adding a capacitor to the sensor circuit. That is, if you don't mind the circuit working a little slower. It may allow you to read the display before the next update occurs.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: Hold reading on LCD meter readout

Post by Edd » Tue May 27, 2003 9:19 am

Karl:
What are you doing…taking micro-Lux readings in a bat cave ?
First of all I would suspect the manuf to be utilizing a dedicated/ custom chip design in the instruments design infrastructure . Thus precluding the access to appropriate internal circuitry points to accomplish your desired mod.
Second would you accept the mechanical possibility of the utilization of one of the mini size keychain fob type of lights that is hardly larger than the small batt that powers it. It typically will contain the batt(s) a pressure switch and a micro incand lamp/or/LED. Since your meter is utilizing a reflective LCD element you should then be able to cut a sheet of ~ 1/8 in thick clear Lucite (Plexiglas) a bit larger than the overall LCD faces proper and then draw an outline on the center portion of the plastic just a bit larger than the numbers display area. Then all of the area outwards from that internal perimeter needs to be wrapped in a shiny side of aluminum foil with its outer surface black paint/taped/masked off…your choice, as well as the edges At one edge of the plastic it is unmasked, where you side inject the beam from the aforementioned light source into the plastic blank. The end result will then be the selective bathing of your LCD element in light, with the masking holding off undesired upward and lateral dispersion of unwanted illumination.
Lastly,if the design Genies are smiling at you, break open the unit and find the U/p circuitry and then locate its timebase xtal/or/cer resonator and then fire up the unit and get a display and then put a short across that element and see if your units display doesn’t just freeze on its current display data. I have seen many instruments and clock displays that will function in that manner.
If that is the case, then it will be a simple manner to mount a small low profile “tact”style of sw (salvageable from a defunct computer, instrument,TV,VCR,Clock panel, etc) right above the clock osc and that would give a hold to freeze the reading.You don’t want anything more than micro lead lead length interconnects though or squirrely things could happenen in U/p action…..by undesired stray pickups.

73's de Edd
eddw35 attbi.com...................(Interstellar~~~~~~~Warp~~~Speed)
[email protected]......(Firewalled*Spam*Cookies*Crumbs)
;)<p>[ May 27, 2003: Message edited by: Edd Whatley ]</p>

Karl Cohen
Posts: 2
Joined: Mon May 26, 2003 1:01 am
Contact:

Re: Hold reading on LCD meter readout

Post by Karl Cohen » Tue May 27, 2003 1:49 pm

Many thanks to all who offered their help.<p>I think I'll go with either the "grain of wheat" lamp or the LED. The other mods suggested are very interesting but way beyond my capabilities.<p>Karl

Dimbulb
Posts: 324
Joined: Sun Oct 13, 2002 1:01 am

Re: Hold reading on LCD meter readout

Post by Dimbulb » Wed May 28, 2003 1:45 pm

Darkness testing uses sample and hold.
At times there can arise a situation where a very small value of light needs to be measured and requires special rigging. I recall using a black cloth and wearing a black T-shirt draping this cloth over my head like the old fashioned photographers and exposing only the photo-diode (or simularly CdS cell simultaneous setup mentioned by Chris)outside of this light barrier. By building a very sensitive hold meter even though it was'nt calibrated it did indicate that the small red led I was using to see with was not affecting the reading. <p>Briefly this sample and hold uses an accumulative type circuit that has a sample period that will store the maximum value or peak on a 1 uF capacitor and an op-amp interfaces the capacitor to the meter with negligable drain
(high impedance). This meter needs to be more sensitive than the other meter or scale used. An adjustable gain on the op-amp with 150 times (or more) than the calibrated meter. You could shoot for less than 1% error. The led under the light barrier may have minute light escaping and can be found eventually and you can gather data with confidence that your light shielding has been verified that the led is not having an effect on reading as well as other light noise.<p>So you actually use two meters one showing ambient integrity of the set up using an led. Now you can view the second meter that should give calibrated acurate data.<p>[ May 28, 2003: Message edited by: dimbulb ]</p>

Post Reply

Who is online

Users browsing this forum: No registered users and 44 guests