Design & Build a Stair-Step Waveform Generator

In this project we will design and build a Stair-Step Waveform Generator which gives a 5 volt peak-to-peak output with steps of 1 volt. Both a fixed oscillator and variable oscillator options will be included. A rotary switch will also give access to additional step waveforms.

 

 

A 74HC4017 Johnson Counter is used to drive a diode and resistor divider network to provide a stair-step waveform, giving an amplitude of 3 volt peak-to-peak. This signal is then coupled via a isolating capacitor to a buffer stage and amplifier which is set to give a final output waveform of 5 volt peak-to-peak. Each step is 1 volt.

The image below shows the clock input to the 74HC4017 and the first three output signal Q0, Q1 and Q2. All pulses are 5 volt in amplitude.

As the outputs from the 74HC4017 are feed via isolation diodes the amplitude of the pulses feed to the resistor divider network is about 4.4 volt (due to 0.6 volt diode voltage drop). The common earth return resistor in the divider network is fixed at 1.2K (in the final circuit this is replaced with a 1K in series with a 1K preset so as to provide some setup adjustment). Each of the other resistors in the network are calculated to provide a pulse output across the 1.2K which increment in steps of 0.6 volt to give a final stair-step waveform of 3 volt peak-to-peak.

The resistor values where calculated to be 7.6K, 3.2K, 1.73K, 1K and 560Ω and available resistor of 0.1% tolerance of 7.68K, 3.24K, 1.74K, 1K and 562Ω were used. If you do not have these values the I have found that standard values of 7.5K, 3.3K, 1.8K, 1K and 560Ω (all 1%) work well.

In the original circuit Q9 (pin11) was connected directly to ground. This was later changed (in Version 2) by adding a 10K resistor to ground.

Below are links to version 2 of the schematic and PCB artwork:

http://www.scullcom.com/Stair_Step_Gen_ver2_Schematic.pdf

http://www.scullcom.com/Stair_Step_Gen_ver2_PCB.pdf

The output stage buffer and amplifier uses a NE5532P Dual OP Amp which is powered from a +6 volt and -6 volt split supply which is generated using a TLE2426 “Rail Splitter” IC which produces a virtual earth splitting the 12 volt supply.

The Johnson Counter IC (74HC4017) can be supplied with a clock pulse from a fixed oscillator circuit using a 1MHz Crystal Oscillator module and a 74HC390 divider IC which gives either a 10KHz or 100KHz clock signal or a variable oscillator. The type of oscillator is selected by links of the PCB (which could be replaced with switches if required).

The variable oscillator uses a Linear Technology LTC1799 which is a precision oscillator in a small SOT-23 (5 pin) package. The frequency of the oscillator is simply set by an external resistor which in this project uses a variable preset (1MΩ preset in series with a 100KΩ resistor). The output of this oscillator can also be divided by 1, 10 and 100 by use of pin 4 of the IC; no connection results in ÷10, pin 4 to ground results in ÷1 and pin 4 to V+ (5v) results in ÷100.

Below you see the LTC1799 soldered on to the PCB print side.

Below you see details of the links on the PCB to set both the type of oscillator to use and the frequency:

Parts List

C1, C3, C4, C6, C7, C10, C11 – 100nF
C2 – 2200uF
C5 – 470uF
C8, C9 – 10uF
D1, D2, D3, D4, D5, D6, D7, D8, D9 – 1N4148 (small switching diode)
JP1, JP2 – 2 pin SIL option link with link plug
JP3, JP4, JP5, JP6, JP7, JP8, JP9, JP11 – Wire Links
JP10 – 3 pin SIL option link with link plug
P1 to P6 – PCB terminal pins
R1 – 10K
R2, R10 – 100K
R3, R9, R11, R12 – 1K
R4 – 7.68K (± 0.1%) (RS stock No. 701-7673) or 7.5K (±1%)
R5 – 3.24K (± 0.1%) (RS stock No. 701-7550) or 3.3K (±1%)
R6 – 1.74K (± 0.1%) (RS stock No. 754-8758) or 1.8K (±1%)
R7 – 1K (± 0.1%) (RS stock No. 701-7383) or 1K 1K (±1%)
R8 – 562Ω (± 0.1%) (RS stock No. 701-7305) or 560Ω (±1%)
RV2 – 1M Potentiometer or Preset
RV3 – 1K Preset
RV4 – 10K Preset
S1 – Rotary Switch, 6 Way, Through Hole (RS Components Ltd stock No. 665-196)
SW2 – Single Pole ON-OFF-ON switch or 3 pin SIL option link with link plug
U1 – LTC1799 Linear Technology precision oscillator in SOT-23 (5 pin) package
U2 – LM7812 12 volt regulator
U3 – LM7805 5 volt regulator
U4 – 74HC4017 Johnson decade counter with 10 decoded outputs
U5 – TLE2426 Texas Instruments, Supply Rail Splitter, “virtual ground” in TO-92 package
U6 – NE5532P Texas Instruments, Dual Op Amp
U7 – 74LS390 Texas Instruments 4-stage, Decade, Decade Counter/Divider
XTL1 – 5V 1MHz Crystal Oscillator Modules Through Hole (DIL-14 format) available on eBay from seller ha5ia for £2.98. Link below:
http://www.ebay.co.uk/itm/181176576561?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

1 x 8 pin DIL socket
2 x 16 pin DIL socket
1 x 15v or 18v DC power adapter

Additional Features

This project also provide step waveforms of 1, 2, 3 or 4 steps (as shown below) by use of a rotary switch to connect the master reset on the Johnson Counter IC with different outputs.