diff --git a/capactive_mains_inputs/Makefile b/capactive_mains_inputs/Makefile
new file mode 100644
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--- /dev/null
+++ b/capactive_mains_inputs/Makefile
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+# Makefile to test and plot sections of the curve 
+# to test the interpolation table
+#
+#
+#
+SOURCE = capacitive_mains_inputs2
+TEX_SOURCE = $(SOURCE).tex
+PDF_SOURCE = $(SOURCE).pdf
+
+#
+#
+# Place all .png files here as .dia targets
+#
+# png files mangle text now in dia
+DIA = opto.dia
+DIAJPG = opto.jpg
+
+doc: $(DIAJPG)
+	echo $?
+	gnuplot reactance.gpt 
+	pdflatex $(TEX_SOURCE)
+	pdflatex $(TEX_SOURCE)
+	acroread $(PDF_SOURCE) || evince $(PDF_SOURCE)
+
+bib:	
+	bibtex $(SOURCE)
+
+#%.png:%.dia
+#	echo $?
+#	dia -t png $<
+#	mkdir -p images_sw_doc
+#	echo $< $@
+#	mv $@ images_sw_doc
+#	echo $@
+
+%.jpg:%.dia
+	echo $?
+	dia -t jpg $<
+	mkdir -p images_sw_doc
+	echo "dia to jpg arg list and target" $< $@
+	mv $@ images_sw_doc
+	echo $@
+
+
+#images.tex:
+#	cat begin_images.tex > images.tex
+#	ls images_sw_doc >> images.tex
+#	cat end_images.tex >> images.tex
+#
+
+clean:
+	rm *.pdf
diff --git a/capactive_mains_inputs/capacitive_mains_inputs2.tex b/capactive_mains_inputs/capacitive_mains_inputs2.tex
new file mode 100644
index 0000000..dae5665
--- /dev/null
+++ b/capactive_mains_inputs/capacitive_mains_inputs2.tex
@@ -0,0 +1,189 @@
+ 
+\documentclass[a4paper,10pt]{article}
+\usepackage[utf8x]{inputenc}
+\usepackage{graphicx}
+\usepackage{fancyhdr}
+\usepackage{lastpage}
+\usepackage{color}
+\definecolor{Blue}{rgb}{0.0,0.0,0.7}
+\definecolor{Red}{rgb}{0.7,0.0,0.0}
+\definecolor{Green}{rgb}{0.0,0.5,0.0}
+\usepackage{hyperref}
+\usepackage{ifthen}
+\usepackage{algorithm}
+\usepackage{algorithmic}
+\usepackage{multirow}
+\usepackage{textcomp}
+
+%opening
+\title{Capacitive Mains Inputs; choosing capacitor and resistor combinations for 120 V a.c. 240 V a.c and 50 and 60Hz}
+\author{R.P. clark}
+
+\begin{document}
+
+\maketitle
+
+\begin{abstract}
+Calculations to work out capacitance values to drive an opto-coupler to detect mains voltage
+for 50 to 60 Hz.
+\end{abstract}
+\begin{figure}[h]
+ \centering
+ \includegraphics[width=200pt]{./images_sw_doc/opto.jpg}
+ % opto.jpg: 854x388 pixel, 72dpi, 30.13x13.69 cm, bb=0 0 854 388
+ \caption{Opto-coupled mains input circuit}
+ \label{fig:opto}
+\end{figure}
+
+\section{Opto coupler circuit}
+
+This circuit is used to detect mains voltage via a capacitor and a resistor
+forming a potential divider so that a lower voltage can be used to drive an opto-isolator
+that protects the processor reading the signal.
+
+
+
+\section{Calculations}
+
+A potential divider using a capacitor and a resistor is used to lower mains voltage
+to levels that can drive a typical opto-coupler input ($\approx 2V$).
+
+A potential divider using a capacitor and resistor means
+using the complex identity for the capacitors reactance, $X$.
+
+$$ X = \frac{-j}{\omega C } $$
+ 
+The ${\omega C }$ term is dependent on frequency and is equivalent to $2.\pi.f$ .
+
+Using a potential divider to determine the voltage over the resistor gives:
+
+$$ V_{out} = V_{in} \times \frac{R}{R-\frac{j}{2.\pi.f.C}} $$
+
+
+The equation above leaves a complex divisor.
+To get a complex number as the numerator, the denominator and numerator must be multiplied by
+the  conjugate of the denominator, thus:
+$$\frac{R}{R-\frac{j}{2.\pi.f.C}} \equiv  \frac{R \times \Big({R+\frac{j}{2.\pi.f.C}}\Big) }{\Big({R-\frac{j}{2.\pi.f.C}}\Big) \times \Big({R+\frac{j}{2.\pi.f.C}}\Big) } $$  
+
+This leaves a real number as the denominator, i.e. $ R^2 + {\frac{1}{2.\pi.f.C}}^2$. 
+The resulting complex number, $X$,  
+$$X = \frac{R \times \Big({R+\frac{j}{2.\pi.f.C}}\Big) }{R^2 + {\frac{1}{2.\pi.f.C}}^2}$$ 
+or,
+\begin{equation}
+ X =\frac{R^2 + \Big({R\frac{j}{2.\pi.f.C}}\Big) }{R^2 + {\frac{1}{2.\pi.f.C}}^2} 
+\label{eqn:genpotdivcapres}
+\end{equation}
+
+can now be evaluated for phase and magnitude. Equation~\ref{eqn:genpotdivcapres} can be generally applied to potential dividers
+in figure~\ref{fig:opto}.
+
+
+\subsection{Example calculation}
+
+
+At 50Hz with 240 V a.c. applied, with R at 1000 Ohms and C at 47 nF
+
+$$\frac{1000^2 + \Big({1000\frac{j}{2.\pi.50.47e-9}}\Big) }{R^2 + {\frac{1}{2.\pi.50.47e-9}}^2}$$
+$$\frac{1000^2 + \Big({1000 \times 67726j}\Big) }{1000^2 + {67726}^2}$$
+$$\frac{1000^2 + \Big({67726000j}\Big) }{4.5877 \times 10^9}$$
+
+This gives a complex number $$ \frac{1000^2 + {67726000j} }{4.5877 \times 10^9}$$ i.e. $$(216 \times 10^{-6} + 14.76\times 10^{-3} j ) \;.$$
+This complex number has a magnitude of 0.0147 and an argument of 89.15 degrees (which is expected as most of the reactance comes from the capacitor).
+So with 240 V a.c. applied (RMS) the opto would see a signal with $0.0147*240 = 3.54V (RMS)$
+\clearpage
+\section{ploting the voltage at the opto-coupler}
+
+\begin{figure}[h]
+ \centering
+ \includegraphics[width=400pt]{./RMS_volts_to_opto.png}
+ % RMS_volts_to_opto.png: 640x480 pixel, 72dpi, 22.58x16.93 cm, bb=0 0 640 480
+ \caption{RMS voltage seen at opto-coupler for 50 to 60 Hz range}
+ \label{fig:rmstoopto}
+\end{figure}
+
+
+\clearpage
+
+\subsection{plotting the voltage at the opto-coupler: gnuplot scripts}
+ 
+{ \tiny
+\begin{verbatim}
+########################################################
+#
+p=3.14159265358979323844
+#
+# 47nF
+C=47e-9
+#
+# 1k Ohms
+R=1000
+
+# define complex operator
+j={0,1}
+
+set xlabel "Hertz"
+set ylabel "Resistance"
+
+# x is the frequency
+set xrange[50:60]
+
+# z(x) is the reactance
+z(x)=(j/(2*p*x*C))
+
+# denominator
+d(x)=(R*R+z(x)*z(x))
+
+# numerator
+n(x)=(R*R+R*z(x))
+
+plot abs(z(x)) title "reactance over capacitor"
+!sleep 4
+
+set ylabel "denominator value (abs)"
+plot abs(d(x))
+!sleep 4
+
+set ylabel "numerator value (abs)"
+plot abs(n(x))
+!sleep 4
+
+v(x)=abs((n(x))/(d(x)))
+
+# gives large numbers h(x)=arg((n(x))/(d(x)))
+
+set ylabel "voltage to opto-coupler (RMS)"
+plot 240*v(x) title "240 V a.c", 120*v(x) title "120 V a.c"
+!sleep 4
+
+set terminal png
+set output "RMS_volts_to_opto.png"
+plot 240*v(x) title "240 V a.c", 120*v(x) title "120 V a.c"
+
+#set angles degrees
+#set label "phase change in mains over opto"
+#plot 240*h(x) title "240 V a.c", 120*h(x) title "120 V a.c"
+#!sleep 4
+#
+\end{verbatim}
+
+
+}
+% 
+% 
+% Putting some numbers in this, 47nF for the capacitor, 1k for R and 50 Hz at 240V, means ${2.\pi.f.C} = 14.765 \times 10^{-6}$.
+% 
+% $$ V_{out} = 240 \times \frac{1000}{ 1000 - \frac{j}{14.765 \times 10^{-6}} } $$
+% or
+% % $$ V_{out} = 240 \times \frac{1000}{1000 - j  \times  67.726 \times 10^3 }$$
+% % % 
+% % To get a complex number as the numerator, the denominator and numerator must be multiplied by
+% % its conjugate, thus:
+% % % 
+% % $$\frac{1000}{1000 - {j}  \times  67.726 \times 10^3 } $$
+% % % $$
+% % % 
+% %  \equiv   \frac{ 1000 \times  (1000 + {j}  \times  67.726 \times 10^3) }{ (1000 - {j}  \times  67.726 \times 10^3) \times (1000 + {j}  \times  67.726 \times 10^3)}   $$
+% % $$
+% 
+typeset in {\Huge \LaTeX} \today.
+ \end{document}
diff --git a/capactive_mains_inputs/opto.dia b/capactive_mains_inputs/opto.dia
new file mode 100644
index 0000000..0d3849e
Binary files /dev/null and b/capactive_mains_inputs/opto.dia differ
diff --git a/capactive_mains_inputs/reactance.gpt b/capactive_mains_inputs/reactance.gpt
new file mode 100644
index 0000000..3c99da8
--- /dev/null
+++ b/capactive_mains_inputs/reactance.gpt
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+########################################################
+#
+p=3.14159265358979323844
+# 47nF
+C=47e-9
+# 1k Ohms
+R=1000
+
+# define complex operator
+j={0,1}
+
+set xlabel "Hertz"
+set ylabel "Resistance"
+
+# x is the frequency
+set xrange[50:60]
+
+# z(x) is the reactance
+z(x)=(j/(2*p*x*C))
+
+# denominator
+d(x)=(R*R+z(x)*z(x))
+
+# numerator
+n(x)=(R*R+R*z(x))
+
+plot abs(z(x)) title "reactance over capacitor"
+!sleep 4
+
+set ylabel "denominator value (abs)"
+plot abs(d(x))
+!sleep 4
+
+set ylabel "numerator value (abs)"
+plot abs(n(x))
+!sleep 4
+
+v(x)=abs((n(x))/(d(x)))
+
+# gives large numbers h(x)=arg((n(x))/(d(x)))
+
+set ylabel "voltage to opto-coupler (RMS)"
+plot 240*v(x) title "240 V a.c", 120*v(x) title "120 V a.c"
+!sleep 4
+
+set terminal png
+set output "RMS_volts_to_opto.png"
+plot 240*v(x) title "240 V a.c", 120*v(x) title "120 V a.c"
+
+#set angles degrees
+#set label "phase change in mains over opto"
+#plot 240*h(x) title "240 V a.c", 120*h(x) title "120 V a.c"
+#!sleep 4
+#