What does the project present?
This vintage DIY electronic kit is a time relay, with a time delay of at least 165 seconds, indicated (at that time) to signal the 3 minutes of a telephone call being exceeded. This 3-minute delay was used because at that time (1988), if this time was exceeded, additional costs were imposed. Of course, the assembly has various other applications in electronics and not only, being relatively easy to build.
We specify that one of the methods of making a time relay is the charging of a capacitance at a constant voltage or current and the comparison of the voltage at its terminals with a reference voltage.
When the two voltages are equalized, an output stage will be ordered to signal optically, acoustically or by means of a relay, meaning the end of the timing. By changing the voltage, charging current, reference voltage or capacitance, it will also change duration of timing.
How does the assembly work?
following electronic scheme presented in the article, we will find the above elements, as follows:
The resistor R1 and the Zenner diode D1 form a stabilization stage, which ensures the constant voltage necessary for charging the capacity C1. With the help of the voltage divider formed by the resistors R2 and R3, the reference voltage is obtained;
Transistor T1 has the role of comparator, noticing the moment when the voltage at the capacitor terminals applied in the base, exceeds the reference voltage, amplified in the emitter, with a value of approx. 0.7V, ie the base-emitter voltage for opening a silicon transistor. Transistor T3 acts as the output stage, controlling the lighting of the LED1, which indicates the end of the preset timing.
At the beginning of the opening T1, a small bias current appears for T3, which through the resistor R6, controls the opening of T2. As soon as T2 enters the conduction, it will parallel the resistors R3 and R5, thus decreasing the reference voltage applied in T1's emitter.
As a result, the base-emitter voltage increases, T1 enters saturation, as does T3 and so LED1 (red) will light up brightly, creating the feeling of an instantaneous opening. LED2 (green) indicates the commissioning of the installation.
Unloading capacity for a new timing cycle, it is done with the help of switch K1.
Technical characteristics of the assembly
- Supply voltage: between 6V and 12 Vdc
- Duration: minimum 165s
- Current absorbed from the source: max. 50mA
List of required components (with recent equivalents):
- T1, T2 - SNC1 transistors (SNC2, BC 170-172) or BC 546 / BC 547 / BC 548
- T3 - transistor SPC1 (SPC2, BC 250 - 252) or BC 556 / BC 557 / BC 559
- D1 - Zenner diode - PL5V1Z, PL4V7Z or 1N4733A / BZX55C5V1 / BZX55C5V1-YAN
- D2 - red LED
- D3 - Rectifier diode
- D4 - green LED
- C1 - capacitor - 100uF / 6.3V
- R1 - resistor - 680 Ohm (min. 0.25W)
- R2 - resistor - 4.7 KOhm (min. 0.25W)
- R3 - resistor - 10 KOhm (min. 0.25W)
- R4 - resistor - 100 KOhm (min. 0.25W)
- R5 - resistor - 10 KOhm (min. 0.25W)
- R6 - resistor - 820 Ohm - 1 KOhm (min. 0.25W)
- R7 - resistor - 470 Ohm (min. 0.25W)
- R8 - resistor - 820 Ohm - 1 KOhm (min. 0.25W)
- RV1 - semi-adjustable 1M
- Switch
- Printed wiring or breadboard test board
- Tin or connecting threads
Download the original IPRS Baneasa SME-8607 leaflet
For a better understanding of the operation of the circuit we will need electronic scheme presented below:
In order to carry out this project in our own laboratory, we will also need printed wiring PCB layout From lower:
Images with assembled montage
Many of you are probably wondering how this DIY electronic kit was packaged or distributed. Below I have attached some pictures with packaged product (new) and the contents of the envelope. Thank you Mr. Marius Balauta for providing pictures!
Other sources:
Thank you Mr. George-Alexandru Marinescu for providing the leaflet (leaflet) and pictures with the assembly assembled and functional!
We lived in those times, but we were lucky to be alone on the wire.
As far as I can remember, the figure only worked when paired.
Thanks for the article! I wanted to look for a timer idea but I can only find it with IC 555.
Do this and dig it until you understand it. The text is quite clear and if you catch how it works, you can change it.
Very correct. It is a beautiful, simple assembly and it does its job without problems… it has worked without mistakes for years!
It took years to set up a telephone station, my family managed to get a "coupled" wire. That is, there were two posts on a line, the null wire was tied to the ground. I installed an additional electronic bell that was commercially available behind the house so that my family could hear the phone in the garden and I had a complaint that I changed the line 🙂
Innovative! I wonder how they could detect if the line is changed or not .. hmm…
Well, the team came and made measurements, that the other "couples" complained that they could no longer be heard well or that they could be heard simultaneously, I don't know anymore.
Did you put a 1 micro capacitor in series with the doorbell? If not here is the bug, because at the time of the call the call voltage was no longer 48vac, it was lower. When checking the line a, b with respect to the ground and between them especially the resistance led to zero and cut the call current. It wasn't a serious problem, but what do you do with envious neighbors... That was Romania then as it is now.