That schematic is hard to read, but it looks to me as though you have the transistors connected in an emitter follower configuration.
The voltage on the emitter of the transistor (to which you have the relay coil connected) is going to be around 0.7V below the voltage on the base, you also have a diode in series with the base resistor, so the voltage on the relay coil is likely to be below 4V when the Arduino output is high.
Do your relays really operate at 4V?.
Thanks for your reply JohnLincoln.
What can I do to improve readability of the Schematic? I would like to improve.
I browsed “emitter follower configuration”, but do not fully understand it. However, the two transistors get their pulse from Arduino Pin8 and 9, respectively. Each transistor then energizes one of two relays to give me a polarized pulse to the 24V coil of a mechanical memory of a public clock some 3 meters away. These pulses are 24V, although the coils of the relays are 12V. The incoming power of the circuit has a 1amp slow blow fuse.
The relay coils get 12V from a voltage regulator (not shown in my schematic sent earlier) to reduce the 24V (2 X 12V lead acid batteries in series) to 12V and the transistors connect the relay coils to Ground for the duration of the pulse supplied by Arduino Pin 8 and 9.
The relays have 12V coils, but I do not know what voltage they actually get since the pulse is too short for the digital volt meter. I assumed it is close to 12V and they have a real healthy click.
My concern is that I do not know where to connect diodes D5 and D6 which is shown in the schematic, but not fitted on the board. Are they really needed seeing the 24 volt circuit is totally isolated from the 12V circuit that has the delicate components? Only the contacts of the relays are connected to 24V and when they turn off, the Memory coil is totally isolated from all other components, unless there could be spikes created on ground (which is also disconnected), but I would not know how and where and how to handle that.
The puzzling part is that with the wires to the Memory coil on T1 disconnected the system runs for hours without any fault. The moment I connect even the coil of a 12V relay to T1 then the GPS receiver dies/stalls for a minute or more and then turns on again. It must be a safety built into the GPS receiver.
Any ideas why it does not like the 24V coil to be connected to T1?
I again attach the schematic.
DipTrace Schematic - Parow Polarized Schematic.pdf (21.7 KB)