There are 4 sections to this circuit, repeated 4 times for each function (Play, Record, Rewind, Fast Forward).

The 4 Sections are:

1. Debounce

2. Logic

3. Latch

4. Drive.

Debounce:

When a button on the front control panel is pressed, it sends the input to one channel of a dual Monostable IC (4538) high. This triggers the IC and it's output stays high for a period determined by a resistor/capacitor pair connected to the IC's pins. Therefore, if the button "Bounces", it will not retrigger the Monostable, hence no false triggering. The outputs from the Remote control also go into the Monostable inputs.

Logic:

The logic circuitry is simply an array of "OR" Gates made up of Diodes and a 4071 (quad 2 input OR gate). The output of the gates are connected to the reset inputs of each Latch, and are used to disable a function if another button is pressed. Example, if the Deck is in Play mode, the latch that controls "Play" will be disabled if Record OR Rewind OR FastForward OR Stop are pressed. All latches are reset on system power up by a power on reset circuit.

24/11/2001: After working on a design for a Hydroponics system, which uses a simplified version of this circuit, and discovering a small glitch, I would recommend that all IN914/IN4148 diodes are replaced with 1N4007. This may very well solve the counter glitch problem mentioned later on.

Latch:

The output from the Debounce section goes into a 4013 CMOS IC. This is a "D type Flip Flop". As wired in the circuit, when it receives a pulse at it's input, it's output ("Q") will change state and stay in that state until the latch is reset or another pulse is received at the input. Q will also go low if the IC is reset (via the Logic circuit), or if it receives another pulse at it's input. For example, if you press Play, the deck will go into play mode, if you press Play again, the cassette deck will stop.

A slight difference with the Rewind circuit is the connection to the "NOT Q" output. The two outputs , Q and NOT Q are always opposite in logic levels. The NOT Q output on the Reverse control is connected to the UP/DOWN input of the digital counter. When Reverse is pressed, the Q output of the 4013 in that circuit goes high, and it's NOT Q output goes low, therefore putting the Counter into countdown mode.

Drive:

The Q output from each 4013 goes into a MOSFET which then provides the current to drive the Solenoids and LED's. Notice that the Record and Play Latches both go, via a simple Diode OR gate, to the same MOSFET to drive the Play solenoid, plus the output from the record latch also turns on another MOSFET which activates the record logic in the existing Philips circuitry. The LED indicator for Play and Record is a dual LED in one package, which glows green in Play mode and red in Record mode.

Motor on/off - How it Works.

This is perhaps the simplest circuit in the system. A MOSFET to turn the Motor on and off, and 4 diodes wired as a simple OR gate.
When a Q output from the D type flip flop of Play, or Rec, or RW, or FFwd is high, the MOSFET is turned on, hence current is allowed to flow through the motor.

Reset/Stop circuit - How it Works.

This is a wierd Reset system, I hear you say, and yep you'd be right.

The reason it's done this way is due to the counter cicruit and an annoying glitch that occurs when the counter is in count down mode. With out this circuit, if stop is pressed the counter sees that as a command to drop the last digit by 1 count (ie - minus 100, if the count was 989, it would become 889). So what we have to do is disable the counter just before the mechanism is stopped (or effectively reset).

So, when stop is pressed, it enables the 4017 and 555 timer which sends pulses to the 4017. The 4017 counts the pulses and turns one of it's outputs on at a time for each count.
Count one disables the counter, count two resets the mechanism while keeping the counter disabled, count three keeps the counter disabled. On the forth count, the 555 is reset as pin 7 is connected to a MOSFET which disables the 555 when Pin 7 goes high. The cycle only starts again if stop is pressed.

The whole reason that the counter is glitching is probably due to a power supply fluctuation when the system is reset. However not having an oscilloscope, I can't observe what's actually happening, so this is a work around until I buy a much needed new toy - er - Oscilloscope.

24/11/2001: After working on a design for a Hydroponics system, which uses a simplified version of the logic circuit, and discovering a small glitch (a very quick pulse which is long enough to light an LED, but too brief to reset the Flip Flips), I would recommend that all IN914/IN4148 diodes are replaced with 1N4007. This may very well solve the counter glitch problem.

17/5/2005: Circuit replaced with complete MCU solution (see the pages covering the MCU design) - no glitches... :-)