The Puppchen was already under development when the US M1 Bazooka was captured in Tunisia. Initially, the German engineers were interested in a mechanical ignition system, involving a firing pin and a good old percussion cap. This was used on the 8,8cm Raketen Panzer Granat 4312 for the Puppchen, which fired from a closed tube with a breech-block. The captured US M1 Bazooka was fired with a flashlight battery producing the current needed to ignite the rocket motor, a system that was deemed ineffective by the Germans, mostly due to the severe problems this would create in a cold environment like the Eastern Front. But as late as 23 September 1943 they still played with the idea of using a battery, and "Testing with a battery that would work in a cold environment" was given as one of the excuses why the first batch of Panzerschrecks wasn't ready for the frontline. When the first attempts of using a mechanical principle with a percussion ignition failed, they came up with a new idea. The result was a 50/50 solution. A mechanical spring pressured trigger system releases a driving rod. This will in turn hit a "Stoßgenerator" (a punch generator) that will produce the current necessary to ignite the rocket motor. This does away with the need for a battery and is reusable over and over again.

The mechanical firing system

The firing mechanism had three stages or phases. 

1.  At the first stage the firing rod rests against the generator and no springs have been compressed. This is the normal stage for transport and storage. 
2. By pulling the cocking handle to the rear a spring pressured rod is moved forward. In its rearmost position the built in safety catch in the cocking handle will engage with the small eyelet, holding the cocking handle in the rear position as a safety. The rod is held by the cocking handle until it is released. Loading was done at this stage to prevent accidental firing of the rocket. Squeezing the trigger has no effect, as it is not yet "in function" but works as a secondary safety. Note the space between the rod and the generator and the protruding nut in front at this stage.
3. To disengage the safety a small lever inside the handle is pushed down. This releases the cocking handle which will return to the forward position. The rod is now held by the trigger (which also works as a sear). This is the firing stage.

Pressing the trigger will release the rod which will strike against the generator producing a electrical current and the mechanism will return to stage 1. 

 
The 6 pictures above are from a instructional slide series and show the different stages of the mechanical firing system

The electric circuit

A rocket has been inserted in the tube and the loader is connecting the plug on the wooden contact holder to the socket outlet.
This is a staged photo during a test firing, as the loader is standing right behind the weapon!

There is a closed electric circuit  that runs from the generator (Stoßgenerator) to the outlet box (Steckerbuchse) mounted at the rear end of the tube. 

The Stoßgenerator. When the rod from the mechanical firing system hits the metal disc of the generator, an electric impuls is produced.

The Stoßgenerator works on a simple principle. The visible metal disc is held by a rubber disc and will not move much, but the impact shock created by the rod will travel from the metal disc to the movable iron core, sending this one to the rear. On its rearward movement it passes through a copper wire spool and a circular magnet and induces a current. A spring at the rear pushes the iron core back again so that it rests against the metal disc again. Due to this construction all the moveable parts are held inside the closed casing.

The rocket that is loaded into the tube from the rear has a wire protruding at the end which is terminated in a wooden contact holder with a plug. This plug is inserted into the outlet box. The electric current produced by the generator will travel through the wire, via the outlet box and into the rocket engine, igniting this. The return current will travel from the engine through a second wire that is soldered to the rocket body inside the tail fin ring and back into the outlet box through a spring pressured pointed pin that grounds the rocket fin ring once the rocket is inserted. 

Illustrations from the D 1864/1 showing the right and wrong way of rocket loading. The spring pressured grounding-pin is visible to the right

The real thing. Note the pointed grounding pin to the rear.

More pictures from the instructional slide series. The use of the wooden contact holder and the wire grounded inside the tailring are visisble

With the introduction of the RPzB 54/1 a whole new and simplified connection system was adapted, together with new ammunition. These improvements will be discussed in the page concerning theRPzB 54/1.

The firing system in (extreme) detail

The parts for the firing system were mounted after the tube had been painted. The finish appears to be a thin zinc coating. 

The spring that drives the firing rod can be found just above and to the rear of the trigger. It is held in place by a cross pin and a cup washer. When the cup washer is under pressure from the spring the pin will be held in place. The firing rod has a second hole with no apparant function. Most likely this hole was used for a special tool when the cross pin/cup washer/driving spring was installed.

     
A very simple comparison of makers and nuts reveals that the smooth nut was only used by the manufacturer kxs (Schricker & Co). This manufacturer also appears to be the only one that has painted their nuts.

Both the trigger and the cocking handle had their own return springs. There was also a spring built into the cocking handle for the safety catch.

The details and measurements of the separate parts can be studied in the technical drawings by William Bogdan in the pictures above.