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Old 18th Feb 2019, 8:59 pm   #1
Robsradio
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Default Are there any Ex GPO engineers here? Dorman Lamps

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Are there any ex GPO engineers on the forum?
In my quest to get a small collection of 1970s road lamps, I have bought a mid 70s Dorman Smith Trafilamp with P.O embossed on the side together with battery and bulb details. Sadly it is missing the inside part with the bulb, switch and maybe flasher circuit.
My question is.. if anybody used these P.O lamps around works tents, did they flash or were they static lamps? I know the later square based BT lamps were static but this one pre dates these.
If anyone knows please message me, and if anybody has one, or even the inside bulb, switch etc insert please get in touch, it would be nice to restore it to a working state.
I have included a photo to show the model.
Thanks for any help as always
Rob
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Old 20th Feb 2019, 3:52 pm   #2
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

Im not x GPO but seem to remember in my youth the bulbs were 6 volt 0.09 mA small Bayonet Cap style
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Old 20th Feb 2019, 6:34 pm   #3
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

So near, and good recall, they are 5v 0.09A. I assume they went for 5v to allow for the volt drop within the flasher circuit, I guess the static lamp's bulbs were slightly overdriven.
I seem to remember that torches that ran on 2 D cells had a 2.5v bulb, so maybe that was the practice to slightly overdrive them, maybe for a whiter light?
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Old 20th Feb 2019, 7:02 pm   #4
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

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Originally Posted by Robsradio View Post
did they flash or were they static lamps?
In my mis-spent youth I had a collection of road lamps. As far as I remember the Dorman-Smith ones flashed. They were switched on and off by poking a thin object through the hole on the front.
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Old 20th Feb 2019, 8:56 pm   #5
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

One I found severely damaged, back in the mists of time, had a bimetallic flashing bulb.
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Old 20th Feb 2019, 9:34 pm   #6
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

In my mis-spent youth (OK, a 1970s university student) it was common for these lamps to be liberated from roadworks (under cover of darkness, often with alcohol as a mediator), disassembled [access to the batteries was by way of a through-bolt which needed a pin-spanner to remove it before the bottom casing could be lowered] and for one of the pair of 6V batteries to be removed.

The thing was then - still feebly-flashing - nocturnally restored to its original position on the barriers/cones round the hole-in-the-road, and the purloined battery would go on to power a transistor-radio.

The ones I remember had a flash-circuit using two TO92-type plastic-cased transistors.
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Old 20th Feb 2019, 10:16 pm   #7
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

I am impressed by the recollection of these from over 40 years ago, I can remember similar experiences, maybe that is why you rarely see roadworks with lamps around them nowadays!
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Old 20th Feb 2019, 11:23 pm   #8
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

I think I might have seen a couple of those loafing about at work in our stores. Must remember to have a closer look. I have never used them. Most of our stuff doesn't involve roadside works.
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Old 21st Feb 2019, 12:17 am   #9
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

we had quite a few of those DS ones abandoned at my dads works, due to renting out a yard to the contractors building the Blythe Bridge Bypass. I used to raid the PJ996 batteries out of them, 2 in parallel. Behind the pinhole was a push-push table lamp switch. The bulb was a tubular MBC and it would only flash with the correct bulb rating in, due to the simple transistor flash circuit. The duration of the flash was extremely brief and the bulb had a fast-acting filament. Pleasant memories!
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Old 21st Feb 2019, 4:08 pm   #10
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

I reverse-engineered a Dorman Smith flashing lamp once ..... well, after the battery was spent, what else was there to do with it? I lost the diagram I drew a long time ago; but I do remember it used a strange circuit with an NPN and a PNP transistor, not the flip-flop you might expect. The duty cycle was very low; but I suppose it had to be, given the disposable battery power source and the tunsten filament lamp load.

In case anyone doubts that this behaviour is hereditary, my dad nicked one of the old paraffin ones back in the 1970s ..... while we were on holiday somewhere near Weston-Super-Mare ..... and brought it all the way back up the M5, where it lived in the back garden for years!
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Old 21st Feb 2019, 4:38 pm   #11
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

Those NPN/PNP flashers had one timing capacitor and relied on the flash of light from the bulb on the LDR or photo diode for the other part of the timing.
I have got a tatty one of the later yellow paraffin lamps.
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Old 21st Feb 2019, 7:15 pm   #12
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

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Originally Posted by julie_m View Post
I reverse-engineered a Dorman Smith flashing lamp once ..... well, after the battery was spent, what else was there to do with it? I lost the diagram I drew a long time ago; but I do remember it used a strange circuit with an NPN and a PNP transistor, not the flip-flop you might expect. The duty cycle was very low; but I suppose it had to be, given the disposable battery power source and the tunsten filament lamp load.

In case anyone doubts that this behaviour is hereditary, my dad nicked one of the old paraffin ones back in the 1970s ..... while we were on holiday somewhere near Weston-Super-Mare ..... and brought it all the way back up the M5, where it lived in the back garden for years!
They must have been slow to get the latest stuff down in Zummerset! Students (note the third person in this statement ) were nicking the flashing ones in London by then.

An old paraffin one with a red pygmy bulb in it was strategically placed on the roof of the union building so as to be visible from halls of residence to indicate whether the dramatic society tech workshop was occupied or not.
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Old 25th Feb 2019, 8:43 pm   #13
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

The lamp-flasher circuit was always intriguing: an optically-coupled multivibrator.

Bulb lights, shines on photocell, photocell turns bulb off. Include the hysteresis of the photocell, the thermal-lag of the bulb and a bit of capacitance and you've got a nice simple oscillator. The truly-cunning bit [ which surely should have been patented!! ] was to let the photocell also 'see' daylight - so during the day the oscillator was locked-out so saving battery-power.

Musing on this, I recovered a dead modern version from the hedge on my morning dog-walk and disassembled it. Seems they're now made by "UNIPART/Dorman" [ yes, the "Unipart" being the same as the late-and-unlamented 1980s supplier of replacement parts to failing British Leyland cars]. These days the electronics are a couple of SMA transistors and a half-dozen resistors/capacitors/diodes which drive a yellow LED as the light-source. The power-supply is still a (single) spring-terminal 996 battery.

As part of 'idiot-proofing' cunning the place the square battery fits is engineered so that whichever orientation you install it, everything still works: there are 4 contacts for the outer battery-spring and one for the centre. This I like !
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Old 26th Feb 2019, 1:08 pm   #14
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

This is how the interest starts.... be careful ;-)
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Old 26th Feb 2019, 3:58 pm   #15
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

The problem with idiot proofing is that here are some clever idiots, the Keeler opthamascope has a battery that can fit either way round (positive either end, negative on a side spring) made it easy to charge at the bottom and light at the top. So many people asked which way round it went so an arrow was put on it.
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Old 26th Feb 2019, 7:25 pm   #16
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

The torches that I remember that used the 996 battery had a contact plate with a central circular metal contact and a metal ring around it. Again, whichever way you put the battery in, the springs would touch the appropriate contacts.
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Old 26th Feb 2019, 10:07 pm   #17
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

I'm fairly sure the subject of my little impromptu teardown did not have a light sensor in it; I think the transistors might have been wired in a deadly embrace, dumping the capacitor's contents into the lamp filament and resetting it for the next cycle.

I'm probably going to have to try breadboarding a few circuits now, just to see if I can reproduce it .....
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Old 27th Feb 2019, 12:23 am   #18
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

Quote:
Originally Posted by merlinmaxwell View Post
The problem with idiot proofing is that here are some clever idiots, the Keeler opthamascope has a battery that can fit either way round (positive either end, negative on a side spring) made it easy to charge at the bottom and light at the top. So many people asked which way round it went so an arrow was put on it.
https://www.euroenergy.co.uk/keeler-...100-p-531.html
At that price I would want a technician to call and change it for me.
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Old 27th Feb 2019, 1:10 am   #19
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

Quote:
Originally Posted by julie_m View Post
I'm fairly sure the subject of my little impromptu teardown did not have a light sensor in it; I think the transistors might have been wired in a deadly embrace, dumping the capacitor's contents into the lamp filament and resetting it for the next cycle.

I'm probably going to have to try breadboarding a few circuits now, just to see if I can reproduce it .....

I don't remember any light sensor either in the one I reverse engineered around 1973. I can't find my circuit, but this one attached below looks pretty similar.
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Old 28th Feb 2019, 11:13 pm   #20
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Default Re: Are there any Ex GPO engineers here? Dorman Lamps

Quote:
I don't remember any light sensor either in the one I reverse engineered around 1973. I can't find my circuit, but this one attached below looks pretty similar.
The ones I have seen didn't have a light sensor, they were just an NPN and PNP a C and a couple of Rs pretty much like your diagram, simplicity
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