eDrum - Schematics



The eDrum controller consists of PIC16F877, opamp LM324, LCD 16x2 and MAX232. You can download and view the PIC16F877 datasheet for more information on this microcontroller, and here are the datasheets for MAX232 and LM324.

Click to enlarge

The PIC is the real heart of the system. It's responsible for A/D conversion, calculating and sending MIDI data and interacting with the user through the simple LCD + 4 button interface.

The circuit including MAX232 and DB9 connector (labbeled as Serial Out) is for the users of PC without MIDI input, like laptop computers.

Power supply unit has two separate voltage regulators, for digital and analog part of the circuit. A wall transformer with output voltage from 12 to 15V and some 500mA will be sufficient. It is connected to J2.

If the LCD has backlight, then the values of the current limiting resistors can be adjusted for optimal brightness (normally 47..100 ohms).

NOTE for firmware up to v1.2 ONLY: Digital inputs from switches are connected to J4 and ground. The purpose of jumper JMP1 is to allow pull-up or pull-down input lines, depending on type of the switch used.

Analog inputs from piezo triggers need some more complicated electronic circuit, as you can see on the schematics below.

Click to enlarge

I choose LM324 because of it's capability to work with signals from GND (including 0V) to something below Vcc (Vcc-1.4V). To achive 0-5V output for full ADC range, LM324 has a separated power supply of 8V.

The piezo sensors are connected to the input jacks with shielded cable. The purpose of the gain pots P1..8 is to match the sensibility of pads, as they can be different and produce different output voltage.

The use of the schottky diode BAT85 is VERY important, as the circuit doesn't work correctly without it (actually, it will work, but with much less sensitivity).

Output from this stage (J9) is connected to J3 and J4 on the digital board.

You may have noticed that there are some strange pins labeled as "AGND". It is conection to the "analog ground" that is separated from GND "digital ground", for better signal-to-noise ratio. eDrum is actually using 10 bit resolution A/D, and separate grounds for analog and digital parts of the circuit are needed. This is something to worry about when making the PCB layout.

The analog and digital ground must be connected together in one single point, in order to avoid ground loops. The best way to do this is to connect pin 2 of 78L05 with pin 2 of 78L08.

Component list

NOTE: Always check the latest schematics for correct part values.

Bill Of Materials
Design:   eDrum Controller
Doc. no.: 1
Revision: 1.3
Author:   Admir Salahovic
Modified: 08/07/05

QTY  PART-REFS                     VALUE
---  ---------                     -----
2    R1,R2                         220
3    R3,R16,R21                    1k
3    R4,R15,R17                    10k
12   R5-R14,R18,R19                100
1    R20                           2k2

2    C1,C2                         22pF
1    C3                            1000uF
10   C4,C5,C7,C8,C10-C15           100nF
2    C6,C9                         1uF

Integrated Circuits
1    U1                            PIC16F877-20
1    U2                            78L05
1    U3                            78L08
1    U4                            MAX232
1    U5                            4051

3    D1,D4,D5                      LED
2    D2,D3                         1N4148

1    BR1                           B40C500
1    J1                            DIN 5B 180 F
1    J2                            CONN-H2
2    J3,J4                         CONN-H10
3    J5,J7A,J7B                    CONN-H4
2    J6A,J6B                       CONN-H16
1    J7                            DB9-F
1    L1                            LINK
1    LCD1                          16x2
1    RV1                           10k Lin
4    S1-S4                         PUSHBUTTON
1    X1                            20MHz

Design:   Analog board
Revision: 0.6
Modified: 01/05/03

QTY  PART-REFS                     VALUE
---  ---------                     -----
8    R1,R11,R21,R31,R41,R51,R61,   47k
16   R2,R3,R12,R13,R22,R23,R32,    100k
8    R4,R14,R24,R34,R44,R54,R64,   1k

16   C1,C2,C11,C12,C21,C22,C31,    100nF
2    C101,C102                     1uF

Integrated Circuits
2    U1,U2                         LM324

8    D1,D11,D21,D31,D41,D51,D61,   BAT85
8    D2,D12,D22,D32,D42,D52,D62,   1N4148

8    J1,J2,J3,J4,J5,J6,J7,J8       JACK NC
1    J9                            SIL10
8    P1,P2,P3,P4,P5,P6,P7,P8       100k

If you have questions regarding the circuit, please post them in the forum.