<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>011 - Disclosure - Note 5 - Trade Receivables</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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      &lt;font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;(5)&amp;#160;&amp;#160;TRADE

      RECEIVABLES&lt;/font&gt;

    &lt;/div&gt;&lt;br/&gt;&lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;

      &lt;font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"&gt;We

      grant credit to our customers, but generally do not require

      collateral or any other security to support amounts due.

      Trade receivables of $27,148,000 at June 30, 2013, and

      $21,438,000 at September 30, 2012, were net of allowances of

      $467,000 and $44,000, respectively. As of June 30, 2013,

      allowances of $467,000 consisted of a $25,000 allowance for

      doubtful accounts and a $442,000 allowance for sales

      returns.&amp;#160;&amp;#160;The allowances at September 30, 2012 of

      $44,000 were for sales returns.&lt;/font&gt;

    &lt;/div&gt;&lt;br/&gt;&lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="justify"&gt;

      &lt;font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"&gt;We

      generally warrant that the products sold by us will be free

      from defects in materials and workmanship for a period of one

      year or less following delivery to our customer. Upon

      determination that the products sold are defective, we

      typically accept the return of such products and refund the

      purchase price to our customer. We record a provision against

      revenue for estimated returns on sales of our products in the

      same period that the related revenues are recognized. We base

      the allowance on historical product returns, as well as

      existing product return authorizations. The following table

      reconciles the changes in our allowance for sales returns

      under warranties:&lt;/font&gt;

    &lt;/div&gt;&lt;br/&gt;&lt;table cellpadding="0" cellspacing="0" width="100%" style=""&gt;

        &lt;tr&gt;

          &lt;td valign="bottom" width="17%" style="BORDER-BOTTOM: black 1pt solid"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;September&amp;#160;30,

              2012&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="31%" style="BORDER-BOTTOM: black 1pt solid"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;Increases

              in the Allowance&amp;#160;Related to&lt;/font&gt;

            &lt;/div&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;Warranties&amp;#160;Issued&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="25%" style="BORDER-BOTTOM: black 1pt solid"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;Reductions

              in the Allowance for Returns Under Warranties&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="19%" style="BORDER-BOTTOM: black 1pt solid"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;June&amp;#160;30,&lt;/font&gt;

            &lt;/div&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt; FONT-WEIGHT: bold"&gt;2013&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

        &lt;/tr&gt;

        &lt;tr style="background-color: #CCEEFF;"&gt;

          &lt;td valign="top" width="17%"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;$44&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="bottom" width="31%"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;$1,401&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="25%"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;$(1,003)&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="3%"&gt;

            &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;&amp;#160;&lt;/font&gt;

          &lt;/td&gt;

          &lt;td valign="top" width="19%"&gt;

            &lt;div style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt" align="center"&gt;

              &lt;font style="DISPLAY: inline; FONT-FAMILY: times new roman; FONT-SIZE: 10pt"&gt;$442&lt;/font&gt;

            &lt;/div&gt;

          &lt;/td&gt;

        &lt;/tr&gt;

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