Incandescent indicator light turn on / turn off time for PDP-8 - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: January 26, 2017, 11:22:47 pm
content: Hi Everyone,So I've got one of Oscar's PDP-8 replicas:http://obsolescence.wixsite.com/obsolescence/pidp-8Looking to tweak the code to display more accurately, but I have some questions about PDP-8/I indicator lamps. I looked at some datasheets at Digikey but didn't see any information about how long in milliseconds it takes to go from fully off to fully on, or vise versa when power is applied or removed. Does anyone have any data or thoughts on this?Thanks,Alan

author: IanB
author_rank: Super Contributor
timestamp: January 26, 2017, 11:56:17 pm
content: If you know what kind of lamp was used, buy a similar one and measure it? I'm sure any pilot lamp with a similar voltage and current rating will have a similar on/off time.

author: danadak
author_rank: Super Contributor
timestamp: January 27, 2017, 02:37:58 am
content: Put one on bench, drive it, look at a photocell with a bias to measurelight level, scope would be good for this. You could take scope gate output,drive a transistor which then drives the lamp. If you do not have a photo-cell cut the top off a metal can NPN transistor and use that as a photocell.Regards, Dana.

author: alank2
author_rank: Super Contributor
timestamp: January 27, 2017, 03:37:58 am
content: Sounds like a good idea for some fun testing - I'll have to do it as soon as I get some free time!

author: calexanian
author_rank: Super Contributor
timestamp: January 27, 2017, 04:06:08 am
content: They light up faster than they usually go dim.

author: IanB
author_rank: Super Contributor
timestamp: January 27, 2017, 04:15:47 am
content: Quote from: calexanian on January 27, 2017, 04:06:08 amNaturally. The time constant for lighting up is governed by how fast the external supply can pump energy into the filament. The time constant for going dim is governed by how fast the filament can radiate heat into the surroundings. We can deduce that the external supply can feed energy in faster than it can be radiated away or the bulb would never light up. Therefore the time constant for lighting up will be faster than the time constant for going dark.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2017, 04:49:58 am
content: It seems like I've seen a quote of 20 milliseconds somewhere but I can't swear to it. I would bet that any small low voltage incandescent indicator lamp will probably get you close enough that nobody would ever notice a lack of authenticity. If you are really insistent on absolute perfection then you'll have to look up precisely what type of indicator lamps the original machine used and find one. A photodiode connected to a scope will let you measure the time constants, a DSO would be easiest but if you connect the lamp to a flasher circuit you could probably get by with an analog scope.

author: calexanian
author_rank: Super Contributor
timestamp: January 27, 2017, 05:21:31 am
content: No matter how much tech advances, everything always comes back to blinky lights.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: January 27, 2017, 01:13:28 pm
content: Quote from: calexanian on January 27, 2017, 05:21:31 amNothing wrong with that. I've been doing a lot of blinky lights over the past 15 years. Got a few blinky LED controllers on my bench at the moment. As in: 140W type of blinky LEDs

author: Seekonk
author_rank: Super Contributor
timestamp: January 27, 2017, 02:55:12 pm
content: I remember boot loading the PDP-8 with the front panel switches. Is it that hard to get the real thing? I knew if I kept my S-100 stuff long enough I would be back in style. My couple hundred new Z8 chips might find a market some day. Do they preheat the lamps with minimum current to make them last longer/faster response?

author: LaserSteve
author_rank: Super Contributor
timestamp: January 27, 2017, 03:48:27 pm
content: Depends if it is a vacuum lamp or a gas filled lamp. Gas filled dims quicker... Hence their use for animated signs.Steve

author: SeanB
author_rank: Super Contributor
timestamp: January 28, 2017, 09:22:18 pm
content: They used a 5V lamp, and ran it with a 470R preheat resistor and with the driver transistor dropping 1v so the lamp ran at around 4V and so lasted longer. Turn on time is around 80-100ms, and turn off time is around 200ms, though this is for older filament 7 segment displays, though the indicator lamps for the digital data that were also there used a similar scheme to drive them. They did not flicker on 50Hz half wave rectified AC, unlike a LED.To emulate this you do not need the preheat, just a RC circuit and a logic level small signal mosfet operated in linear region to control the current in the led. Run the modern led's at around 5-10mA, otherwise they are way too bright to be real.

author: alank2
author_rank: Super Contributor
timestamp: January 28, 2017, 09:38:04 pm
content: Are you sure about the voltage Sean? I read a page about the control panel that said they used:http://www.digikey.com/products/en?keywords=CM2187-NDWhich is 28V !

author: SeanB
author_rank: Super Contributor
timestamp: January 28, 2017, 10:08:54 pm
content: Not sure, but used and fixed equipment of around the same age ( including a HP mini that also had blinkenlights) and that was a typical driver for them. Using 28v lamps with some preheat and running at 24V is also possible, but in making an emulation it would be best to use a lower voltage so you do not have 1W resistors dumping excess heat in the board.I got into the habit of checking every new lamp at 3v for lighting before use, from a few 5V lamps being mixed into the bin of 28V ones. They do glow very bright for a split second in a 28V circuit. Then checked them for continuity as well, just so they were not actually blown. Resistance measurements would not be too conclusive, cold resistance varied too much with ambient on those lamps.

author: alank2
author_rank: Super Contributor
timestamp: February 03, 2017, 04:13:03 pm
content: I have a question - to make LED's look evenly bright, for example 16 LED's wide, 16 levels of brightness, I would have to apply a gamma correction. Once that is done, it looks very nice. Do you have to do the same thing with incandescent bulbs? If you light one with 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, will they look like even? 10% would be a duty cycle using a transistor driver for example.

author: artag
author_rank: Super Contributor
timestamp: February 04, 2017, 11:49:44 am
content: Should you light it at the same speed as the original ? It's smaller, after all.Model trains run well above scale speed, but not, I think, at actual speed.Small animal's hearts beat faster than large animals

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2017, 06:24:56 pm
content: You probably will have a curve, as the led light output will be somewhat more linear than the filament lamp, which shows a distinct dimming with lower light output, so the half voltage point will be around 30% brightness.however if all are LED, then they will all have the same relative brightness, though to emulate a lamp you would likely have to use a RC integrator and a somewhat linear drive circuit to get it to both have the slow response and emulate dimming. PWM drive to a RC filter and a simple NPN voltage follower with the LED and series resistor, probably driven off a slightly higher voltage rail ( and a few level shifters to change the 3V3/5v drive to the rail voltage) like 9V ( so there is a larger linear range) so the resistor will set full brightness and still have a large linear range as the PWM varies the base voltage.